Merge branch 'main' of https://git.naps.pt/naps62/maestro into oboe

# Conflicts:
#	web/src/catalog.ts

CLAUDE.md

@@ -36,10 +36,16 @@ optional.
 
 ## Where the work is
 
-Linear project **Maestro** under team **Naps62**.
+Gitea issues on [`naps62/maestro`](https://git.naps.pt/naps62/maestro), grouped
+into milestones.
 
-`.claude/linear.json` wires `/yolo` and `/work` to this project — they pick from
-"available now" (status backlog/pending, not blocked).
+`.claude/linear.json` (legacy filename, `tracker: gitea`) wires `/yolo` and
+`/work` to that tracker — they pick from open, unassigned issues whose
+dependencies are all closed. `/blitz <milestone>` sweeps a whole milestone.
+
+Labels carry one value per axis: `area/*`, `type/*`, `difficulty/*`.
+`difficulty` drives model selection downstream, so it tracks the shape of the
+work, not its size.
 
 ## Repo layout
 
@@ -72,6 +78,10 @@ before changing anything that creates or removes one.
 sidebars — line layout, activity colours, duration format, state precedence.
 Read it before changing sidebar rendering.
 
+`docs/PREVIEW.md` is the contract for what the content pane shows while the
+operator moves the sidebar highlight — what a preview may touch, what ends it,
+what commits it. Read it before changing sidebar-driven navigation.
+
 `docs/RESTART.md` is the contract for what clients do when the daemon restarts
 or is upgraded under them — the build stamp on `/api/info`, the status
 indicator's states, the blocking overlay, grace and poll timings. Read it

crates/maestro-daemon/src/activity.rs

@@ -69,7 +69,7 @@ pub fn decide(idle_for: Duration, awaiting_input: bool) -> ActivityState {
 /// How long a session's pane must be continuously focused (a client's pane,
 /// not a sidebar highlight) before an `AwaitingInput` glow clears: the
 /// operator has looked, so the rail stops asking for attention.
-pub const FOCUS_CLEAR_HOLD: Duration = Duration::from_secs(5);
+pub const FOCUS_CLEAR_HOLD: Duration = Duration::from_secs(1);
 
 /// Downgrade `raw` from `AwaitingInput` to `Idle` once the pane has been
 /// focused for at least [`FOCUS_CLEAR_HOLD`].
@@ -161,7 +161,7 @@ mod tests {
     fn awaiting_input_glows_until_focus_hold_is_reached() {
         let now = std::time::Instant::now();
         let mut suppressed = false;
-        let since = now - (FOCUS_CLEAR_HOLD - Duration::from_secs(1));
+        let since = now - (FOCUS_CLEAR_HOLD - Duration::from_millis(200));
         assert_eq!(
             apply_focus_suppression(
                 ActivityState::AwaitingInput,

crates/maestro-tui/src/app.rs

@@ -11,7 +11,6 @@
 //! `dispatch` / the overlay routing.
 
 use std::collections::{HashMap, HashSet};
-use std::io::Stdout;
 use std::path::PathBuf;
 use std::time::Duration;
 
@@ -19,7 +18,6 @@ use chrono::Utc;
 use crossterm::event::{Event as CtEvent, EventStream};
 use futures_util::StreamExt;
 use maestro_protocol::{routes, BranchInfo, CreateSessionRequest, DaemonClient, NameBatch};
-use ratatui::backend::CrosstermBackend;
 use ratatui::Terminal;
 use uuid::Uuid;
 
@@ -89,8 +87,52 @@ pub struct App {
     /// that can move `Focus::Pane` (attach, tab switch, sidebar toggle,
     /// mouse) is covered without each one remembering to report it.
     focused_session: Option<Uuid>,
+    /// A sidebar highlight waiting out its debounce before it becomes a
+    /// preview (`docs/PREVIEW.md`). Walking the list with `j` held down must
+    /// not open an attach per row it passes over.
+    pending_preview: Option<(Uuid, std::time::Instant)>,
+    /// Sessions attached only because they were previewed, oldest first. Kept
+    /// connected past the preview so stepping back onto a row is instant, and
+    /// bounded so a long walk doesn't leave a socket per session behind.
+    preview_lru: Vec<Uuid>,
+    /// Fingerprint of the last frame drawn. The loop compares a freshly
+    /// computed key against this and skips the repaint when they match — an
+    /// idle TUI used to emit a full diff 30 times a second forever, which over
+    /// a remote terminal is constant traffic for no change on screen.
+    last_frame: Option<FrameKey>,
+    /// When that frame went out, for the periodic refresh below.
+    last_drawn_at: std::time::Instant,
+    /// Bumped by every path that mutates `AppState` outside the catalog and
+    /// the attached pane: keys, mouse, draft results, link transitions. Those
+    /// mutations are too diffuse to fingerprint individually, so the loop
+    /// treats "an input was processed" as "assume the frame is stale".
+    epoch: u64,
 }
 
+/// Everything a frame is built from that is cheap to sample every tick.
+/// Equal keys mean an identical frame, so the repaint can be skipped.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+struct FrameKey {
+    catalog: u64,
+    epoch: u64,
+    /// Active pane's session, its screen revision, and its link state.
+    pane: Option<(Uuid, u64, crate::session_task::SessionState)>,
+    area: ratatui::layout::Size,
+}
+
+/// Session cards carry a duration that advances on its own (`docs/SIDEBAR.md`),
+/// so a frame is never allowed to stand for longer than this even when nothing
+/// else changed.
+const CLOCK_REFRESH: Duration = Duration::from_secs(1);
+
+/// How long a highlight has to stand still before the pane previews it
+/// (`docs/PREVIEW.md`). Long enough that a fast walk down the list doesn't
+/// attach to every row it crosses, short enough to read as immediate.
+const PREVIEW_DELAY: Duration = Duration::from_millis(120);
+
+/// How many preview-only attaches to keep connected behind the current one.
+const PREVIEW_BUDGET: usize = 3;
+
 impl std::fmt::Debug for App {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("App")
@@ -170,13 +212,18 @@ impl App {
             last_sent_size: HashMap::new(),
             clicks: ClickTracker::default(),
             focused_session: None,
+            pending_preview: None,
+            preview_lru: Vec::new(),
+            last_frame: None,
+            last_drawn_at: std::time::Instant::now(),
+            epoch: 0,
         }
     }
 
     /// Drive the loop until the user quits. Consumes self.
-    pub async fn run(
+    pub async fn run<B: ratatui::backend::Backend>(
         mut self,
-        terminal: &mut Terminal<CrosstermBackend<Stdout>>,
+        terminal: &mut Terminal<B>,
     ) -> Result<(), TuiError> {
         if let Ok(info) = self.daemon.daemon_info().await {
             self.home_dir = info.home_dir;
@@ -184,7 +231,7 @@ impl App {
         let mut events = EventStream::new();
         loop {
             self.sync_focus().await;
-            self.redraw(terminal)?;
+            self.redraw_if_stale(terminal)?;
 
             tokio::select! {
                 Some(Ok(ev)) = events.next() => {
@@ -193,17 +240,24 @@ impl App {
                     }
                 }
                 Some(update) = self.draft_rx.recv() => {
+                    self.epoch = self.epoch.wrapping_add(1);
                     self.apply_draft_update(update).await;
                 }
                 _ = self.catalog.changed() => {
-                    // Render on next loop iteration.
+                    // Picked up by the frame key's catalog version.
                 }
                 _ = self.link.changed() => {
                     // The status indicator and the restart overlay both read
-                    // the link; redraw on the next loop iteration.
+                    // the link; neither is fingerprinted, so mark stale.
+                    self.epoch = self.epoch.wrapping_add(1);
                 }
                 _ = tokio::time::sleep(Duration::from_millis(33)) => {
-                    let effects = self.poll_due_probes_at(std::time::Instant::now());
+                    let now = std::time::Instant::now();
+                    let mut effects = self.poll_due_probes_at(now);
+                    effects.extend(self.poll_due_preview_at(now));
+                    if !effects.is_empty() {
+                        self.epoch = self.epoch.wrapping_add(1);
+                    }
                     self.perform(effects).await;
                 }
             }
@@ -217,6 +271,10 @@ impl App {
             return Ok(false);
         };
         let key = self.clicks.promote(key, std::time::Instant::now());
+        // Any key can move the cursor, open an overlay or edit a text buffer.
+        // Rather than fingerprint each of those, treat a processed input as
+        // proof the frame is stale.
+        self.epoch = self.epoch.wrapping_add(1);
 
         // The daemon is gone: everything a key could do goes through it, so
         // only the quit binding still resolves until it answers again.
@@ -294,6 +352,7 @@ impl App {
                 self.state.sidebar_highlight = Some(SidebarItem::Session(id));
             }
         }
+        self.refresh_preview();
         true
     }
 
@@ -415,8 +474,15 @@ impl App {
                         if let Some(previous) = self.state.active_session() {
                             self.release_session(previous);
                         }
-                        effects.push(Effect::Attach(sid));
+                        // A previewed session is already attached: committing
+                        // to it keeps that stream rather than reopening one.
+                        if self.emulators.get(sid).is_none() {
+                            effects.push(Effect::Attach(sid));
+                        }
                     }
+                    // It is the pane's own session now, not a loan from the
+                    // preview budget.
+                    self.preview_lru.retain(|s| *s != sid);
                     self.state.attach_to_focused(sid);
                 }
             }
@@ -443,9 +509,89 @@ impl App {
                 }
             }
         }
+        // Every action can move the highlight or the focus, and the preview is
+        // a function of both (`docs/PREVIEW.md`).
+        self.refresh_preview();
         effects
     }
 
+    /// Recompute what the pane should be previewing. A session highlighted in
+    /// the sidebar is previewed after [`PREVIEW_DELAY`]; anything else — a
+    /// group row, the pane holding the keyboard, or the highlight already
+    /// sitting on the attached session — clears the preview at once.
+    fn refresh_preview(&mut self) {
+        let wanted = match (self.state.focus, &self.state.sidebar_highlight) {
+            (Focus::Sidebar, Some(SidebarItem::Session(sid))) => Some(*sid),
+            _ => None,
+        };
+        match wanted {
+            Some(sid) if self.state.active_session() != Some(sid) => {
+                if self.state.preview != Some(sid) {
+                    self.pending_preview = Some((sid, std::time::Instant::now() + PREVIEW_DELAY));
+                }
+            }
+            _ => {
+                self.pending_preview = None;
+                self.clear_preview();
+            }
+        }
+    }
+
+    /// The debounce elapsed: show the highlighted session, attaching it if
+    /// nothing already is.
+    fn poll_due_preview_at(&mut self, now: std::time::Instant) -> Vec<Effect> {
+        let Some((sid, due)) = self.pending_preview else {
+            return Vec::new();
+        };
+        if now < due {
+            return Vec::new();
+        }
+        self.pending_preview = None;
+        if self.state.preview == Some(sid) {
+            return Vec::new();
+        }
+        self.state.preview = Some(sid);
+        // Set outside the input path, so nothing else marks the frame stale.
+        self.epoch = self.epoch.wrapping_add(1);
+        let fresh = self.emulators.get(sid).is_none();
+        if fresh {
+            self.preview_lru.retain(|s| *s != sid);
+            self.preview_lru.push(sid);
+        }
+        self.trim_preview_budget();
+        if fresh {
+            vec![Effect::Attach(sid)]
+        } else {
+            Vec::new()
+        }
+    }
+
+    fn clear_preview(&mut self) {
+        if self.state.preview.take().is_some() {
+            self.epoch = self.epoch.wrapping_add(1);
+        }
+        self.trim_preview_budget();
+    }
+
+    /// Drop preview-only attaches past the budget. What a tab holds, and what
+    /// is on screen right now, are never candidates.
+    fn trim_preview_budget(&mut self) {
+        while self.preview_lru.len() > PREVIEW_BUDGET {
+            let held: HashSet<Uuid> = self
+                .state
+                .tabs
+                .iter()
+                .filter_map(crate::state::Tab::session)
+                .chain(self.state.preview)
+                .collect();
+            let Some(pos) = self.preview_lru.iter().position(|sid| !held.contains(sid)) else {
+                break;
+            };
+            let victim = self.preview_lru.remove(pos);
+            self.release_session(victim);
+        }
+    }
+
     /// Open a stream for `sid` if one isn't already running.
     fn attach(&mut self, sid: Uuid) {
         if self.emulators.get(sid).is_some() {
@@ -959,9 +1105,46 @@ impl App {
         self.focused_session = wanted;
     }
 
-    fn redraw(
+    /// Sample everything a frame depends on. Cheap: two atomic loads, an
+    /// `RwLock` read of a `u64`, and the backend's size.
+    fn frame_key(&self, area: ratatui::layout::Size) -> FrameKey {
+        let pane = self.state.displayed_session().and_then(|sid| {
+            self.emulators
+                .get(sid)
+                .map(|h| (sid, h.revision(), h.state()))
+        });
+        FrameKey {
+            catalog: self.catalog.version(),
+            epoch: self.epoch,
+            pane,
+            area,
+        }
+    }
+
+    /// Repaint only when the frame key moved, or the clock refresh is due.
+    fn redraw_if_stale<B: ratatui::backend::Backend>(
         &mut self,
-        terminal: &mut Terminal<CrosstermBackend<Stdout>>,
+        terminal: &mut Terminal<B>,
+    ) -> Result<(), TuiError> {
+        // Polled rather than driven off `Event::Resize`: `Terminal::draw`'s own
+        // autoresize only runs on a draw, and a skipped draw must still notice
+        // the terminal changed shape.
+        let area = terminal
+            .size()
+            .map_err(|e| TuiError::Terminal(e.to_string()))?;
+        let key = self.frame_key(area);
+        if self.last_frame == Some(key) && self.last_drawn_at.elapsed() < CLOCK_REFRESH {
+            return Ok(());
+        }
+        self.redraw(terminal)?;
+        self.last_frame = Some(key);
+        self.last_drawn_at = std::time::Instant::now();
+        Ok(())
+    }
+
+    fn redraw<B: ratatui::backend::Backend>(
+        &mut self,
+        terminal: &mut Terminal<B>,
     ) -> Result<(), TuiError> {
         let snap = self.catalog.snapshot();
         let attached: HashSet<Uuid> = self
@@ -975,21 +1158,31 @@ impl App {
             highlight: self.state.sidebar_highlight.clone(),
             attached,
         };
-        let active_sid = self.state.active_session();
-        let pane = active_sid.and_then(|sid| {
+        let shown_sid = self.state.displayed_session();
+        let previewing = self.state.preview.is_some();
+        let pane = shown_sid.and_then(|sid| {
             self.emulators.get(sid).map(|h| Pane {
                 screen: h.screen.clone(),
                 status: PaneStatus::from(h.state()),
-                focused: matches!(self.state.focus, Focus::Pane),
+                // A preview never owns the keyboard, so it never draws a
+                // cursor either (`docs/PREVIEW.md`).
+                focused: matches!(self.state.focus, Focus::Pane) && !previewing,
             })
         });
-        let pane_title = match active_sid {
-            Some(sid) => snap
-                .sessions
-                .iter()
-                .find(|session| session.id == sid)
-                .map(crate::sidebar::name_for)
-                .unwrap_or_else(|| "session".into()),
+        let pane_title = match shown_sid {
+            Some(sid) => {
+                let name = snap
+                    .sessions
+                    .iter()
+                    .find(|session| session.id == sid)
+                    .map(crate::sidebar::name_for)
+                    .unwrap_or_else(|| "session".into());
+                if previewing {
+                    format!("{name}  ·  preview")
+                } else {
+                    name
+                }
+            }
             None => "session".into(),
         };
 
@@ -1017,7 +1210,7 @@ impl App {
         // Resize propagation: record the active pane's rect, recompute the
         // per-dimension min for every attached session, send Resize when
         // changed.
-        if let (Some(sid), Some(rect)) = (active_sid, self.region.pane) {
+        if let (Some(sid), Some(rect)) = (shown_sid, self.region.pane) {
             self.emulators.resize_tracker_mut().set_pane_size(
                 sid,
                 "active",
@@ -1257,6 +1450,100 @@ mod tests {
         )
     }
 
+    /// Counts how many times the loop actually repainted, by watching the
+    /// `TestBackend` buffer's generation via `draw` calls.
+    fn drew(app: &mut App, terminal: &mut Terminal<ratatui::backend::TestBackend>) -> bool {
+        let before = app.last_drawn_at;
+        app.redraw_if_stale(terminal).unwrap();
+        app.last_drawn_at != before
+    }
+
+    #[test]
+    fn unchanged_frame_is_not_repainted() {
+        let mut app = make_app();
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(80, 24)).unwrap();
+
+        assert!(drew(&mut app, &mut terminal), "first frame must paint");
+        assert!(
+            !drew(&mut app, &mut terminal),
+            "nothing changed; the repaint should be skipped"
+        );
+    }
+
+    #[test]
+    fn a_processed_key_makes_the_frame_stale() {
+        let mut app = make_app();
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(80, 24)).unwrap();
+        drew(&mut app, &mut terminal);
+
+        app.epoch = app.epoch.wrapping_add(1);
+        assert!(drew(&mut app, &mut terminal));
+    }
+
+    #[test]
+    fn a_catalog_event_makes_the_frame_stale() {
+        let mut app = make_app();
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(80, 24)).unwrap();
+        drew(&mut app, &mut terminal);
+
+        app.catalog
+            .apply_event(maestro_protocol::CatalogEvent::Snapshot {
+                sessions: Vec::new(),
+            });
+        assert!(drew(&mut app, &mut terminal));
+    }
+
+    #[test]
+    fn a_resize_makes_the_frame_stale() {
+        let mut app = make_app();
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(80, 24)).unwrap();
+        drew(&mut app, &mut terminal);
+
+        terminal.backend_mut().resize(120, 40);
+        assert!(drew(&mut app, &mut terminal));
+    }
+
+    #[test]
+    fn pane_output_makes_the_frame_stale() {
+        use crate::session_task::{SessionHandle, SessionState};
+        use std::sync::atomic::{AtomicU64, AtomicU8};
+        use std::sync::{Arc, Mutex};
+
+        let mut app = make_app();
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(80, 24)).unwrap();
+
+        let sid = Uuid::from_u128(7);
+        let revision = Arc::new(AtomicU64::new(0));
+        app.emulators.insert(
+            sid,
+            SessionHandle {
+                tx: tokio::sync::mpsc::channel(1).0,
+                screen: Arc::new(Mutex::new(vt100::Parser::new(24, 80, 0))),
+                state: Arc::new(AtomicU8::new(SessionState::Live as u8)),
+                revision: revision.clone(),
+            },
+        );
+        app.state.attach_to_focused(sid);
+        drew(&mut app, &mut terminal);
+        assert!(!drew(&mut app, &mut terminal), "idle pane must not repaint");
+
+        revision.fetch_add(1, std::sync::atomic::Ordering::Release);
+        assert!(drew(&mut app, &mut terminal), "new PTY bytes must repaint");
+    }
+
+    #[test]
+    fn a_stale_clock_repaints_even_with_no_events() {
+        let mut app = make_app();
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(80, 24)).unwrap();
+        drew(&mut app, &mut terminal);
+        assert!(!drew(&mut app, &mut terminal));
+
+        // Session cards show an advancing duration, so a frame may not stand
+        // for longer than CLOCK_REFRESH regardless of the key.
+        app.last_drawn_at = std::time::Instant::now() - CLOCK_REFRESH;
+        assert!(drew(&mut app, &mut terminal));
+    }
+
     #[test]
     fn enter_insert_then_leave() {
         let mut app = make_app();
@@ -1332,6 +1619,165 @@ mod tests {
         assert_eq!(app.state.overlay(), Some(&OverlayKind::Spawn));
     }
 
+    /// Seed the catalog with `ids` as sessions in one directory.
+    fn app_with_sessions(ids: &[Uuid]) -> App {
+        use chrono::Utc;
+        use maestro_protocol::{CatalogEvent, Session, SessionStatus};
+        let app = make_app();
+        app.catalog.apply_event(CatalogEvent::Snapshot {
+            sessions: ids
+                .iter()
+                .map(|id| Session {
+                    id: *id,
+                    command: "bash".into(),
+                    cwd: std::path::PathBuf::from("/"),
+                    env: Default::default(),
+                    metadata: Default::default(),
+                    worktree: None,
+                    status: SessionStatus::Running,
+                    created_at: Utc::now(),
+                    foreground: None,
+                    activity: Default::default(),
+                    activity_detail: None,
+                    last_activity_at: Utc::now(),
+                    activity_since: chrono::Utc::now(),
+                    current_cwd: None,
+                    blocked_work: None,
+                })
+                .collect(),
+        });
+        app
+    }
+
+    fn fake_handle() -> crate::session_task::SessionHandle {
+        use crate::session_task::{SessionHandle, SessionState};
+        use std::sync::atomic::{AtomicU64, AtomicU8};
+        use std::sync::{Arc, Mutex};
+        SessionHandle {
+            tx: tokio::sync::mpsc::channel(1).0,
+            screen: Arc::new(Mutex::new(vt100::Parser::new(24, 80, 0))),
+            state: Arc::new(AtomicU8::new(SessionState::Live as u8)),
+            revision: Arc::new(AtomicU64::new(0)),
+        }
+    }
+
+    fn after_debounce() -> std::time::Instant {
+        std::time::Instant::now() + PREVIEW_DELAY + Duration::from_millis(1)
+    }
+
+    /// docs/PREVIEW.md: the highlight shows a session in the pane without the
+    /// tab taking it on.
+    #[test]
+    fn highlighting_a_session_previews_it_once_the_debounce_elapses() {
+        let a = Uuid::from_u128(1);
+        let mut app = app_with_sessions(&[a]);
+        app.dispatch(Action::SidebarNext);
+
+        // Nothing yet: the highlight has to stand still first.
+        assert_eq!(app.poll_due_preview_at(std::time::Instant::now()), vec![]);
+        assert_eq!(app.state.preview, None);
+
+        assert_eq!(
+            app.poll_due_preview_at(after_debounce()),
+            vec![Effect::Attach(a)]
+        );
+        assert_eq!(app.state.preview, Some(a));
+        assert_eq!(app.state.displayed_session(), Some(a));
+        // The tab is untouched — the preview is a view, not a layout change.
+        assert_eq!(app.state.active_tab().session(), None);
+    }
+
+    /// A fast walk down the list attaches to where it stopped, not to every
+    /// row it crossed.
+    #[test]
+    fn walking_past_rows_only_previews_the_one_it_lands_on() {
+        let a = Uuid::from_u128(1);
+        let b = Uuid::from_u128(2);
+        let mut app = app_with_sessions(&[a, b]);
+        app.dispatch(Action::SidebarNext);
+        app.dispatch(Action::SidebarNext);
+
+        let effects = app.poll_due_preview_at(after_debounce());
+        assert_eq!(effects, vec![Effect::Attach(b)]);
+        assert_eq!(app.state.preview, Some(b));
+    }
+
+    /// The pane's own session is not something to preview.
+    #[test]
+    fn highlighting_the_attached_session_clears_the_preview() {
+        let a = Uuid::from_u128(1);
+        let b = Uuid::from_u128(2);
+        let mut app = app_with_sessions(&[a, b]);
+        app.state.attach_to_focused(a);
+        app.state.focus = Focus::Sidebar;
+
+        app.state.sidebar_highlight = Some(SidebarItem::Session(b));
+        app.refresh_preview();
+        app.poll_due_preview_at(after_debounce());
+        assert_eq!(app.state.preview, Some(b));
+
+        app.state.sidebar_highlight = Some(SidebarItem::Session(a));
+        app.refresh_preview();
+        assert_eq!(app.state.preview, None);
+        assert_eq!(app.state.displayed_session(), Some(a));
+    }
+
+    /// Moving off every session — onto a group row, or into the pane — puts
+    /// back what the tab was showing.
+    #[test]
+    fn leaving_the_session_rows_ends_the_preview() {
+        let a = Uuid::from_u128(1);
+        let b = Uuid::from_u128(2);
+        let mut app = app_with_sessions(&[a, b]);
+        app.state.attach_to_focused(a);
+        app.state.focus = Focus::Sidebar;
+        app.state.sidebar_highlight = Some(SidebarItem::Session(b));
+        app.refresh_preview();
+        app.poll_due_preview_at(after_debounce());
+
+        app.dispatch(Action::FocusPane(crate::action::Direction::Right));
+        assert_eq!(app.state.preview, None);
+        assert_eq!(app.state.displayed_session(), Some(a));
+    }
+
+    /// Committing keeps the stream the preview already opened.
+    #[test]
+    fn attaching_to_a_previewed_session_reuses_its_stream() {
+        let a = Uuid::from_u128(1);
+        let mut app = app_with_sessions(&[a]);
+        app.dispatch(Action::SidebarNext);
+        app.poll_due_preview_at(after_debounce());
+        // Stand in for the attach the effect would have opened.
+        app.emulators.insert(a, fake_handle());
+
+        let effects = app.dispatch(Action::AttachHighlighted);
+        assert_eq!(effects, vec![]);
+        assert_eq!(app.state.active_tab().session(), Some(a));
+        assert_eq!(app.state.preview, None);
+        // No longer on loan from the preview budget.
+        assert!(!app.preview_lru.contains(&a));
+    }
+
+    /// Preview-only attaches are kept for a step back, but bounded.
+    #[test]
+    fn preview_attaches_are_dropped_past_the_budget() {
+        let ids: Vec<Uuid> = (1..=(PREVIEW_BUDGET as u128 + 2))
+            .map(Uuid::from_u128)
+            .collect();
+        let mut app = app_with_sessions(&ids);
+        for id in &ids {
+            app.state.sidebar_highlight = Some(SidebarItem::Session(*id));
+            app.refresh_preview();
+            app.poll_due_preview_at(after_debounce());
+            app.emulators.insert(*id, fake_handle());
+        }
+
+        assert_eq!(app.preview_lru.len(), PREVIEW_BUDGET);
+        // The oldest rows previewed are gone; the newest are still warm.
+        assert!(app.emulators.get(ids[0]).is_none());
+        assert!(app.emulators.get(*ids.last().unwrap()).is_some());
+    }
+
     /// Seed the catalog with one session and highlight it.
     fn app_with_highlighted_session(sid: Uuid) -> App {
         use chrono::Utc;
@@ -2091,6 +2537,7 @@ mod tests {
             tx: mpsc::channel(1).0,
             screen: Arc::clone(&screen),
             state: Arc::new(AtomicU8::new(SessionState::Live as u8)),
+            revision: Arc::new(std::sync::atomic::AtomicU64::new(0)),
         };
         let mut app = make_app();
         app.state.active_tab_mut().pane = Some(sid);

crates/maestro-tui/src/catalog.rs

@@ -87,6 +87,16 @@ impl CatalogStore {
     }
 
     /// Cheap O(sessions) read of current state.
+    /// The version counter alone, without cloning the session list. The render
+    /// loop reads this every tick to decide whether the sidebar is stale.
+    pub fn version(&self) -> u64 {
+        self.inner
+            .state
+            .read()
+            .expect("catalog rwlock poisoned")
+            .version
+    }
+
     pub fn snapshot(&self) -> CatalogSnapshot {
         let s = self.inner.state.read().expect("catalog rwlock poisoned");
         let mut sessions: Vec<Session> = s.sessions.values().cloned().collect();

crates/maestro-tui/src/config.rs

@@ -52,10 +52,11 @@ const DEFAULT_NORMAL: &[(&str, &str)] = &[
     // because Normal mode only exists while the sidebar is focused, so
     // nothing forwards it to a pty.
     ("tab", "AttachHighlighted"),
-    ("s", "OpenSpawnModal"),
-    // Fast-forward spawn (#67): the spawn key's neighbour does the same
+    // `n`/`N`, the same pair the web client binds.
+    ("n", "OpenSpawnModal"),
+    // Fast-forward spawn (#67): the spawn key's shifted twin does the same
     // thing with every default resolved and no form.
-    ("S", "FastForwardSpawn"),
+    ("N", "FastForwardSpawn"),
     ("x", "KillHighlighted"),
     ("r", "RenameHighlighted"),
     // Focus
@@ -220,11 +221,11 @@ mod tests {
             Some(Action::AttachHighlighted),
         );
         assert_eq!(
-            cfg.keymap.resolve(Mode::Normal, "s"),
+            cfg.keymap.resolve(Mode::Normal, "n"),
             Some(Action::OpenSpawnModal),
         );
         assert_eq!(
-            cfg.keymap.resolve(Mode::Normal, "S"),
+            cfg.keymap.resolve(Mode::Normal, "N"),
             Some(Action::FastForwardSpawn),
         );
     }
@@ -311,18 +312,18 @@ mod tests {
         assert!(s.contains("ctrl-glorb"), "diagnostic: {s}");
     }
 
-    /// Regression for #70: the previous canonical form of a real Shift+S
-    /// keypress was `"shift-S"`, which the default `"S"` binding never
+    /// Regression for #70: the previous canonical form of a real Shift+N
+    /// keypress was `"shift-N"`, which the default `"N"` binding never
     /// matched. Drive the actual crossterm event, not the binding string,
     /// so the dead-key bug can't come back unnoticed.
     #[test]
-    fn real_shift_s_keypress_resolves_to_fast_forward_spawn() {
+    fn real_shift_n_keypress_resolves_to_fast_forward_spawn() {
         use crossterm::event::{Event as CtEvent, KeyCode, KeyEvent, KeyModifiers};
 
         use crate::input::{canonicalise, Region};
 
         let cfg = TuiConfig::load(&TuiSection::default()).unwrap();
-        let ev = CtEvent::Key(KeyEvent::new(KeyCode::Char('S'), KeyModifiers::SHIFT));
+        let ev = CtEvent::Key(KeyEvent::new(KeyCode::Char('N'), KeyModifiers::SHIFT));
         let key = canonicalise(&ev, |_, _| Region::Unknown).expect("key event canonicalises");
 
         assert_eq!(

crates/maestro-tui/src/emulators.rs

@@ -168,6 +168,7 @@ mod tests {
             tx: mpsc::channel(1).0,
             screen: Arc::new(Mutex::new(vt100::Parser::new(24, 80, 0))),
             state: Arc::new(AtomicU8::new(SessionState::Live as u8)),
+            revision: Arc::new(std::sync::atomic::AtomicU64::new(0)),
         };
         emulators.insert(id, handle);
         assert!(emulators.get(id).is_some());

crates/maestro-tui/src/input.rs

@@ -597,7 +597,8 @@ mod tests {
     /// so a real Shift+S delivers `Char { c: 'S', shift: true }`. Its
     /// canonical form must be the bare `"S"` a keymap binds, not `"shift-S"` —
     /// the uppercase letter already says "shifted"; a redundant prefix would
-    /// make the default `S` binding unreachable from the keyboard (#70).
+    /// make every uppercase binding (the default `N`) unreachable from the
+    /// keyboard (#70).
     #[test]
     fn shift_flag_is_redundant_for_an_already_uppercase_char() {
         let k = InputKey::Char {

crates/maestro-tui/src/lib.rs

@@ -24,6 +24,7 @@ pub mod state;
 pub mod terminal;
 pub mod text_input;
 
+use std::io::BufWriter;
 use std::path::PathBuf;
 use std::time::Duration;
 
@@ -121,13 +122,23 @@ pub async fn run(cfg: RunConfig) -> Result<(), TuiError> {
     let _guard = terminal::TerminalGuard::enter().map_err(|e| match e {
         terminal::TerminalError::Io(io) => TuiError::Terminal(io.to_string()),
     })?;
-    let backend = CrosstermBackend::new(std::io::stdout());
+    // Buffered: ratatui issues one `write!` per cell run — thousands per
+    // frame — and an unbuffered `Stdout` takes the global stdout lock on every
+    // one of them. A single BufWriter collapses a frame into one flush.
+    let backend = CrosstermBackend::new(BufWriter::with_capacity(
+        FRAME_BUFFER_BYTES,
+        std::io::stdout(),
+    ));
     let mut terminal = Terminal::new(backend).map_err(|e| TuiError::Terminal(e.to_string()))?;
 
     let app = App::new(tui_cfg, catalog, daemon, link);
     app.run(&mut terminal).await
 }
 
+/// One full 200x50 repaint is ~5 KB of escape sequences; 64 KB leaves room for
+/// a large terminal without ever flushing mid-frame.
+const FRAME_BUFFER_BYTES: usize = 64 * 1024;
+
 /// Startup probe budget, applied to connect *and* the whole request: the user is
 /// staring at a terminal, so an unreachable daemon must fail in under a second.
 const HEALTH_TIMEOUT: Duration = Duration::from_millis(500);

crates/maestro-tui/src/session_task.rs

@@ -14,7 +14,7 @@
 //!   parser is **not** reset on reconnect — the last-known frame stays
 //!   visible until new bytes arrive, so the user doesn't see a blank.
 
-use std::sync::atomic::{AtomicU8, Ordering};
+use std::sync::atomic::{AtomicU64, AtomicU8, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
@@ -56,11 +56,15 @@ pub enum SessionState {
 ///   here.
 /// - `state` — `Arc<AtomicU8>`: live status of the WS, queryable without
 ///   holding the screen lock.
+/// - `revision` — `Arc<AtomicU64>`: bumped every time the parser's screen is
+///   mutated. The render loop compares it against the last drawn frame's so an
+///   idle pane costs a `load` instead of a repaint.
 #[derive(Clone)]
 pub struct SessionHandle {
     pub tx: mpsc::Sender<SessionInput>,
     pub screen: Arc<Mutex<vt100::Parser>>,
     pub(crate) state: Arc<AtomicU8>,
+    pub(crate) revision: Arc<AtomicU64>,
 }
 
 impl std::fmt::Debug for SessionHandle {
@@ -81,6 +85,12 @@ impl SessionHandle {
         }
     }
 
+    /// Monotonic counter of screen mutations. Only equality across two reads
+    /// is meaningful — the loop uses it to decide whether a frame is stale.
+    pub fn revision(&self) -> u64 {
+        self.revision.load(Ordering::Acquire)
+    }
+
     /// Stop the underlying task. Used when the pane closes and the user has
     /// no other tabs pointing at this session.
     pub fn end(&self) {
@@ -109,9 +119,11 @@ impl SessionTask {
         let (tx, mut rx) = mpsc::channel::<SessionInput>(64);
         let parser = Arc::new(Mutex::new(vt100::Parser::new(rows, cols, SCROLLBACK_ROWS)));
         let state = Arc::new(AtomicU8::new(SessionState::Connecting as u8));
+        let revision = Arc::new(AtomicU64::new(0));
 
         let parser_bg = parser.clone();
         let state_bg = state.clone();
+        let revision_bg = revision.clone();
 
         tokio::spawn(async move {
             let url = ws_url;
@@ -151,6 +163,7 @@ impl SessionTask {
                                 }
                                 SessionInput::Resize { cols, rows } => {
                                     parser_bg.lock().expect("vt100 parser mutex").set_size(rows, cols);
+                                    revision_bg.fetch_add(1, Ordering::Release);
                                     let frame = serde_json::to_string(&ClientControl::Resize {
                                         cols,
                                         rows,
@@ -166,6 +179,7 @@ impl SessionTask {
                             match frame {
                                 Some(Ok(Message::Binary(bytes))) => {
                                     parser_bg.lock().expect("vt100 parser mutex").process(&bytes);
+                                    revision_bg.fetch_add(1, Ordering::Release);
                                 }
                                 Some(Ok(Message::Text(text))) => {
                                     // Ping/Pong control frames travel as text JSON. Reply to Ping.
@@ -202,6 +216,7 @@ impl SessionTask {
             tx,
             screen: parser,
             state,
+            revision,
         }
     }
 }
@@ -228,6 +243,7 @@ mod tests {
             tx: mpsc::channel(1).0,
             screen: Arc::new(Mutex::new(vt100::Parser::new(24, 80, 0))),
             state: Arc::new(AtomicU8::new(SessionState::Connecting as u8)),
+            revision: Arc::new(AtomicU64::new(0)),
         };
         assert_eq!(h.state(), SessionState::Connecting);
     }
@@ -238,8 +254,26 @@ mod tests {
             tx: mpsc::channel(1).0,
             screen: Arc::new(Mutex::new(vt100::Parser::new(24, 80, 0))),
             state: Arc::new(AtomicU8::new(SessionState::Live as u8)),
+            revision: Arc::new(AtomicU64::new(0)),
         };
         h.end();
         assert_eq!(h.state(), SessionState::Ended);
     }
+
+    #[test]
+    fn revision_starts_at_zero_and_tracks_screen_mutations() {
+        let revision = Arc::new(AtomicU64::new(0));
+        let h = SessionHandle {
+            tx: mpsc::channel(1).0,
+            screen: Arc::new(Mutex::new(vt100::Parser::new(24, 80, 0))),
+            state: Arc::new(AtomicU8::new(SessionState::Live as u8)),
+            revision: revision.clone(),
+        };
+        assert_eq!(h.revision(), 0);
+
+        // Stands in for the WS pump: process bytes, then bump.
+        h.screen.lock().unwrap().process(b"hello");
+        revision.fetch_add(1, Ordering::Release);
+        assert_eq!(h.revision(), 1);
+    }
 }

crates/maestro-tui/src/state.rs

@@ -63,6 +63,11 @@ pub struct AppState {
     pub mode: Mode,
     pub sidebar_visible: bool,
     pub sidebar_highlight: Option<SidebarItem>,
+    /// The session the pane is *previewing* (`docs/PREVIEW.md`): shown because
+    /// the sidebar highlight is on it, not because a tab holds it. It never
+    /// enters `tabs`, is never reported to the daemon as focused, and is
+    /// dropped the moment the highlight moves on.
+    pub preview: Option<Uuid>,
     /// Group paths whose cards are hidden. This is UI state rather than
     /// catalog state, so it survives every catalog redraw.
     pub collapsed_sidebar_groups: HashSet<PathBuf>,
@@ -91,6 +96,7 @@ impl AppState {
             mode: Mode::Normal,
             sidebar_visible: true,
             sidebar_highlight: None,
+            preview: None,
             collapsed_sidebar_groups: HashSet::new(),
             overlay: None,
         }
@@ -126,6 +132,9 @@ impl AppState {
     /// drop straight into Insert mode so keystrokes go to the session
     /// without an extra `i` press. The leader returns to Normal.
     pub fn attach_to_focused(&mut self, session_id: Uuid) {
+        // Committing to what was previewed: the pane owns it now, so the
+        // preview has nothing left to say.
+        self.preview = None;
         self.active_tab_mut().pane = Some(session_id);
         self.focus = Focus::Pane;
         self.mode = Mode::Insert;
@@ -150,6 +159,13 @@ impl AppState {
         self.active_tab().session()
     }
 
+    /// What the pane actually draws: the preview when one is up, otherwise the
+    /// active tab's session. A preview covers the pane without replacing it,
+    /// so ending one gives back exactly what was there (`docs/PREVIEW.md`).
+    pub fn displayed_session(&self) -> Option<Uuid> {
+        self.preview.or_else(|| self.active_session())
+    }
+
     /// Move the sidebar highlight one step through the rendered item
     /// sequence, in the direction of `delta` (positive = down), landing only
     /// on items `is_stop` accepts. Only the sign of `delta` matters. Wraps.

docs/INSTALL.md

@@ -141,7 +141,8 @@ chord_timeout_ms = 1000      # how long to hold a partial chord like `ctrl-w`
 
 [tui.keymap.normal]
 "i"            = "EnterInsert"
-"s"            = "OpenSpawnModal"
+"n"            = "OpenSpawnModal"
+"N"            = "FastForwardSpawn"
 "x"            = "KillHighlighted"
 "r"            = "RenameHighlighted"
 "?"            = "ShowHelp"

docs/PREVIEW.md

@@ -0,0 +1,71 @@
+# PREVIEW.md — the sidebar preview contract
+
+One rule for what the content pane shows while the operator is only moving
+around the sidebar, so the web client and the TUI behave the same. Read this
+before changing either. If the behaviour needs to change, change it here first.
+
+## The rule
+
+Highlighting a session in the sidebar shows that session in the content pane,
+as if it were focused, **without focusing it and without changing the layout**.
+Moving the highlight elsewhere puts back exactly what was on screen before.
+
+A preview is a view, not a state. Nothing about it is durable:
+
+- The layout is byte-for-byte identical before the preview and after it ends.
+- Nothing is persisted while a preview is up (the web client's layout is saved
+  to `localStorage`; a preview never reaches it).
+- The daemon is never told the previewed pane is focused, so a preview never
+  clears a session's `AwaitingInput` glow (`docs/SIDEBAR.md`).
+- The keyboard stays in the sidebar: the previewed terminal is on screen but
+  does not take focus, so the next `j`/`k` still walks the list.
+
+## Where the preview is shown
+
+| The previewed session is… | What happens |
+|---|---|
+| Already in a pane, in the active tab | That pane comes forward (its tab's focus moves to it). |
+| Already in a pane, in another tab | That tab is brought forward. The active tab is restored when the preview ends. |
+| Not in any pane | A throwaway tab carries it. It is removed when the preview ends. |
+
+The throwaway tab never counts as an open session: the tab bar does not appear
+because of it, the mobile shell does not switch away from the session list for
+it, and the sidebar does not mark the session as open.
+
+## What ends a preview
+
+- The highlight moves to another session — that session is previewed instead.
+- The highlight leaves every session (a folder row, or the list losing the
+  keyboard, or the pointer leaving the list): the previous view comes back.
+- Any real layout change (split, close, new tab, tab switch, drag): the preview
+  is discarded first, so a throwaway tab is never baked into the arrangement.
+
+## What commits a preview
+
+Choosing the previewed session — clicking or pressing Enter on its row,
+clicking into its pane, or handing the keyboard to the pane (web: `Tab` from
+the list; TUI: the attach binding) — keeps exactly what is on screen. A
+throwaway tab becomes a real one; a focus move inside an existing tab stands.
+Nothing moves a second time on commit.
+
+## Timing
+
+A preview follows the highlight closely enough to be usable as a way to look
+around, and lags a pointer enough not to flash rows crossed on the way
+somewhere else.
+
+| Trigger | Delay |
+|---|---|
+| Keyboard step (`j`/`k`, arrows) | 60 ms |
+| Pointer hover (web, mouse only — never touch) | 150 ms |
+| Clearing (highlight left every session) | same delay as the trigger that set it |
+
+Touch has no hover, and a tap already opens a session, so touch never previews.
+
+## Cost
+
+A previewed session that no pane holds needs a live attach. Both clients keep
+it connected after the preview ends — the web client parks it in the connection
+pool's LRU, the TUI keeps its emulator until it falls out of the preview
+budget — so stepping back onto a row is instant rather than re-fetching
+scrollback.

docs/SIDEBAR.md

@@ -234,7 +234,7 @@ the agent, and answering 204 would hide it.
 
 A session's pane — the pane a client's own attach view is showing, never a
 sidebar highlight — clears its `AwaitingInput` glow once it has been
-**continuously focused for 5 seconds**: the operator has looked, so the rail
+**continuously focused for 1 second**: the operator has looked, so the rail
 stops asking for attention and the card reads as `Idle` instead. This is a
 real state change, not a client-side paint-over — the daemon computes it, so
 every client watching the session (web, TUI, a second pane on the same
@@ -274,3 +274,21 @@ than jumping to the end of the list.
 The web client needs a chord to reach the list at all (`Ctrl+Shift+H`, and the
 same chord back): an attached terminal owns every plain keystroke. The TUI uses
 `ctrl-w h`.
+
+Everything else is the same key on both clients:
+
+| key     | action                                      |
+|---------|---------------------------------------------|
+| `j`/`k` | move the highlight (arrows too)             |
+| `Enter` | open the session, or expand the folder      |
+| `Tab`   | hand the keyboard to the pane               |
+| `c`     | collapse or expand the highlighted folder   |
+| `r`     | rename the highlighted session              |
+| `x`     | kill the highlighted session                |
+| `n`     | open the spawn form                         |
+| `N`     | spawn with every default resolved           |
+| `?`     | show the shortcut overlay                   |
+
+The TUI binds these through its keymap, so a user can rebind any of them; the
+web client's are fixed. A binding added to one client belongs in this table and
+in both help overlays.

flake.nix

@@ -148,7 +148,7 @@
             pnpmDeps = (pkgs.fetchPnpmDeps.override { pnpm = pkgs.pnpm_10; }) {
               inherit (finalAttrs) pname version src;
               fetcherVersion = 3;
-              hash = "sha256-AvIrpXipbcp7hRujh/L4UAc6tAJOxe67P/iji6L/oL0=";
+              hash = "sha256-hOqiAbnjKsoQnV8lgto5HspDsLIb+48sNLkShIjCZPQ=";
             };
 
             buildPhase = ''
@@ -218,8 +218,8 @@
             settings = {
               # `nix run .#dev` works from a bare shell, not only from inside
               # `nix develop`, so the toolchain is put on PATH here rather than
-              # inherited. `/usr/bin:/bin` stays last for anything else a
-              # spawned session reaches for.
+              # inherited. This covers the build and Vite; the daemon widens it
+              # further for the sessions it spawns (see `daemon.command`).
               environment = [
                 "PATH=${lib.makeBinPath devTools}:/usr/bin:/bin"
                 # Not :7070: that is the daemon's own default, and the dev
@@ -241,7 +241,17 @@
                   # `environment`. A tmux socket of its own keeps dev sessions
                   # out of — and safe from — the tmux server belonging to
                   # whatever daemon the developer actually runs.
+                  #
+                  # A spawned session inherits the daemon's PATH, and the
+                  # toolchain PATH above holds build tools only — on NixOS the
+                  # trailing `/usr/bin:/bin` is all but empty, so every session
+                  # opened from the dev UI came up unable to find `claude`,
+                  # `zsh` or anything else the developer actually has. Append
+                  # the usual user profile directories so dev sessions behave
+                  # like real ones. Resolved by the shell at run time; the ones
+                  # that do not exist on a given host cost nothing.
                   command = ''
+                    PATH="$PATH:$HOME/.nix-profile/bin:/etc/profiles/per-user/$(id -un)/bin:/run/current-system/sw/bin:/usr/local/bin" \
                     MAESTRO_DAEMON_TMUX_TMPDIR="$PWD/.dev/tmux" \
                     RUST_LOG="''${RUST_LOG:-maestro=debug,info}" \
                       target/debug/maestro daemon

web/package.json

@@ -15,6 +15,7 @@
   "dependencies": {
     "@fontsource/jetbrains-mono": "^5.3.0",
     "@xterm/addon-fit": "^0.10.0",
+    "@xterm/addon-webgl": "^0.19.0",
     "@xterm/xterm": "^5.5.0"
   },
   "devDependencies": {

web/pnpm-lock.yaml

@@ -14,6 +14,9 @@ importers:
       '@xterm/addon-fit':
         specifier: ^0.10.0
         version: 0.10.0(@xterm/xterm@5.5.0)
+      '@xterm/addon-webgl':
+        specifier: ^0.19.0
+        version: 0.19.0
       '@xterm/xterm':
         specifier: ^5.5.0
         version: 5.5.0
@@ -348,6 +351,9 @@ packages:
     peerDependencies:
       '@xterm/xterm': ^5.0.0
 
+  '@xterm/addon-webgl@0.19.0':
+    resolution: {integrity: sha512-b3fMOsyLVuCeNJWxolACEUED0vm7qC0cy4wRvf3oURSzDTYVQiGPhTnhWZwIHdvC48Y+oLhvYXnY4XDXPoJo6A==}
+
   '@xterm/xterm@5.5.0':
     resolution: {integrity: sha512-hqJHYaQb5OptNunnyAnkHyM8aCjZ1MEIDTQu1iIbbTD/xops91NB5yq1ZK/dC2JDbVWtF23zUtl9JE2NqwT87A==}
 
@@ -772,6 +778,8 @@ snapshots:
     dependencies:
       '@xterm/xterm': 5.5.0
 
+  '@xterm/addon-webgl@0.19.0': {}
+
   '@xterm/xterm@5.5.0': {}
 
   assertion-error@2.0.1: {}

web/src/attach.ts

@@ -1,4 +1,5 @@
 import { FitAddon } from "@xterm/addon-fit";
+import { WebglAddon } from "@xterm/addon-webgl";
 import { Terminal } from "@xterm/xterm";
 import "@xterm/xterm/css/xterm.css";
 
@@ -10,6 +11,11 @@ import { applyModsToChar, cursorSeq, tildeSeq } from "./keys.js";
 import { requestToggleDrawer } from "./drawer.js";
 import { splitCommand, toolIconSvg } from "./toolIcon.js";
 import type { ClientControl, Uuid } from "./types.js";
+import { WebglPolicy } from "./webglRenderer.js";
+
+/** One budget for the whole page — see webglRenderer.ts for why contexts are
+ *  rationed rather than handed to every connection. */
+const webglPolicy = new WebglPolicy();
 
 /**
  * A single live attach: an xterm terminal bound to the daemon's attach WebSocket.
@@ -26,8 +32,10 @@ export interface AttachConnection {
   readonly sessionId: Uuid;
   /** The `.attach` element (header + terminal body) to mount into a pane. */
   readonly el: HTMLElement;
-  /** Becomes the visible view: fit to the pane, focus, sync size. */
-  onMount(): void;
+  /** Becomes the visible view: fit to the pane, focus, sync size.
+   *  `focus: false` mounts without taking the keyboard — what a sidebar
+   *  preview needs, so j/k keeps working while the pane is on screen. */
+  onMount(opts?: { focus?: boolean }): void;
   /** Parked into the cache: keep streaming, just stop driving focus/resize. */
   onUnmount(): void;
   /** Put the keyboard back into this terminal — what leaving the sidebar does. */
@@ -135,6 +143,72 @@ export function createAttachConnection(
   term.loadAddon(fit);
   term.open(body);
 
+  // GPU rendering is scoped to the mounted view, not the connection: acquired
+  // in onMount, released in onUnmount. See webglRenderer.ts.
+  let webgl: WebglAddon | null = null;
+  let dprMedia: MediaQueryList | null = null;
+
+  // The glyph atlas is rasterised for one device pixel ratio. Dragging the
+  // window to a display with a different one (or an Android browser-zoom
+  // change) leaves every cell blurry until the atlas is rebuilt.
+  const onDprChange = () => {
+    try {
+      term.clearTextureAtlas();
+    } catch {
+      // ignore — nothing to rebuild if the renderer already went away
+    }
+    watchDpr();
+  };
+
+  function watchDpr(): void {
+    dprMedia?.removeEventListener("change", onDprChange);
+    dprMedia = null;
+    try {
+      dprMedia = window.matchMedia?.(`(resolution: ${window.devicePixelRatio}dppx)`) ?? null;
+      dprMedia?.addEventListener("change", onDprChange);
+    } catch {
+      // ignore — a browser without `resolution` media queries just keeps the
+      // atlas it has, which is the pre-WebGL behaviour anyway
+    }
+  }
+
+  function releaseWebgl(): void {
+    dprMedia?.removeEventListener("change", onDprChange);
+    dprMedia = null;
+    if (!webgl) return;
+    const addon = webgl;
+    webgl = null;
+    try {
+      addon.dispose(); // xterm falls back to the DOM renderer
+    } catch {
+      // ignore — a lost context can throw on the way out
+    }
+  }
+
+  function acquireWebgl(): void {
+    if (webgl || disposed || !webglPolicy.enabled()) return;
+    try {
+      const addon = new WebglAddon();
+      // Context loss is routine on Android — backgrounding the tab is enough.
+      // Drop back to the DOM renderer, then retry only while the shared budget
+      // allows and this view is still the one on screen.
+      addon.onContextLoss(() => {
+        webglPolicy.noteContextLoss();
+        releaseWebgl();
+        if (mounted) acquireWebgl();
+      });
+      term.loadAddon(addon);
+      webgl = addon;
+      watchDpr();
+    } catch {
+      // Context creation failed outright (no GPU, or the browser is already at
+      // its context cap). Count it like a loss so we stop asking, and stay on
+      // the DOM renderer.
+      webglPolicy.noteContextLoss();
+      webgl = null;
+    }
+  }
+
   // Plain Escape hands the keyboard back to the session list instead of the
   // shell/app. `false` tells xterm to skip its own handling of this event.
   term.attachCustomKeyEventHandler((e) => {
@@ -237,6 +311,10 @@ export function createAttachConnection(
   }
 
   let mounted = false;
+  // A previewed pane is visible but must not steal the keyboard from the
+  // sidebar, so mounting can opt out of the focus grab (and out of the one
+  // the socket does when it finishes connecting).
+  let focusOnMount = true;
   // We compute the fit ourselves instead of FitAddon.fit(): the addon subtracts
   // a scrollbar width that xterm reports as `offsetWidth - scrollArea || 15`,
   // so with the scrollbar hidden it falls back to a phantom 15px and the grid
@@ -315,7 +393,7 @@ export function createAttachConnection(
       reconnectMs = INITIAL_RECONNECT_MS;
       setState("connected", "open");
       sendControl({ type: "Resize", cols: term.cols, rows: term.rows });
-      if (mounted) term.focus();
+      if (mounted && focusOnMount) term.focus();
     };
 
     ws.onmessage = (ev) => {
@@ -400,17 +478,22 @@ export function createAttachConnection(
   return {
     sessionId,
     el,
-    onMount() {
+    onMount(mountOpts?: { focus?: boolean }) {
       mounted = true;
+      focusOnMount = mountOpts?.focus !== false;
+      acquireWebgl();
       // Defer a tick so the element has its content-pane dimensions before fit.
       requestAnimationFrame(() => {
         tryFit();
         sendControl({ type: "Resize", cols: term.cols, rows: term.rows });
-        if (ws.readyState === WebSocket.OPEN) term.focus();
+        if (focusOnMount && ws.readyState === WebSocket.OPEN) term.focus();
       });
     },
     onUnmount() {
       mounted = false;
+      // Parked views hand their GPU context back: the browser's cap is small
+      // (smaller still on Android) and MAX_PARKED connections outlive the pane.
+      releaseWebgl();
     },
     focusTerminal() {
       term.focus();
@@ -427,6 +510,7 @@ export function createAttachConnection(
       visual?.removeEventListener("resize", onResize);
       dataDisp.dispose();
       resizeDisp.dispose();
+      releaseWebgl();
       try {
         ws.close();
       } catch {

web/src/catalog.ts

@@ -364,6 +364,11 @@ export interface SidebarOptions {
   /** Daemon link behind the footer's status dot. Omitted in tests and in the
    *  design kit, which render the sidebar without a daemon. */
   link?: DaemonLink;
+  /** The highlight moved onto a session, or off every session (`null`): the
+   *  shell shows it in the content pane without opening it (docs/PREVIEW.md).
+   *  `source` is there so a mouse sweep can be debounced differently from a
+   *  keyboard step. */
+  onPreview?: (sessionId: string | null, source: "key" | "hover") => void;
 }
 
 export function renderSidebar(store: CatalogStore, opts: SidebarOptions = {}): SidebarHandle {
@@ -528,6 +533,9 @@ export function renderSidebar(store: CatalogStore, opts: SidebarOptions = {}): S
     for (const r of rows) r.classList.toggle("is-highlighted", r.dataset.navKey === navKey);
   };
 
+  const sessionOf = (key: string | null): string | null =>
+    key?.startsWith("session:") ? key.slice("session:".length) : null;
+
   const moveNav = (delta: number) => {
     const rows = navRows();
     if (rows.length === 0) return;
@@ -539,6 +547,7 @@ export function renderSidebar(store: CatalogStore, opts: SidebarOptions = {}): S
     navKey = next.dataset.navKey ?? null;
     applyNav();
     next.scrollIntoView({ block: "nearest" });
+    opts.onPreview?.(sessionOf(navKey), "key");
   };
 
   /** Enter: open the highlighted session, or expand the highlighted folder. */
@@ -563,9 +572,50 @@ export function renderSidebar(store: CatalogStore, opts: SidebarOptions = {}): S
     applyNav();
   };
 
+  // Hover previews the row under the pointer (docs/PREVIEW.md). Touch is left
+  // out: there is no hover there, and a tap already opens the session.
+  let hovered: string | null = null;
+  const setHovered = (id: string | null) => {
+    if (hovered === id) return;
+    hovered = id;
+    opts.onPreview?.(id, "hover");
+  };
+  treeHost.addEventListener("pointerover", (e) => {
+    if (e.pointerType === "touch") return;
+    const row = (e.target as HTMLElement | null)?.closest<HTMLElement>(".tree-leaf");
+    setHovered(row?.dataset.sessionId ?? null);
+  });
+  treeHost.addEventListener("pointerleave", (e) => {
+    if (e.pointerType === "touch") return;
+    setHovered(null);
+  });
+
   // Focusable as a unit: the walk moves a highlight, not DOM focus, so the
   // keyboard stays here while j/k runs.
   treeHost.tabIndex = -1;
+  // The keyboard walk's preview belongs to the list having the keyboard: once
+  // it doesn't, the content pane goes back to what the operator actually opened.
+  treeHost.addEventListener("blur", () => opts.onPreview?.(null, "key"));
+
+  /** The session behind the highlighted row, if the highlight is on a card. */
+  const highlightedSession = (): { row: HTMLElement; session: Session } | undefined => {
+    const row = rowFor(navKey);
+    const id = row?.dataset.sessionId;
+    if (!row || !id) return undefined;
+    const session = store.snapshot().sessions.get(id);
+    return session ? { row, session } : undefined;
+  };
+
+  /** `c`: collapse or expand the highlighted folder, the TUI's
+   *  `ToggleSidebarGroup`. Clicking the header is what toggles it; on a card
+   *  the key does nothing rather than toggling some other folder. */
+  const toggleHighlightedFolder = () => {
+    const row = rowFor(navKey);
+    if (!row || !row.classList.contains("tree-folder-header")) return;
+    row.click();
+    applyNav();
+  };
+
   treeHost.addEventListener("keydown", (e) => {
     if (e.ctrlKey || e.altKey || e.metaKey) return;
     // The inline rename field lives inside a row and owns its own keys.
@@ -574,7 +624,14 @@ export function renderSidebar(store: CatalogStore, opts: SidebarOptions = {}): S
     else if (e.key === "k" || e.key === "ArrowUp") moveNav(-1);
     else if (e.key === "Enter") activateNav();
     else if (e.key === "Tab") document.dispatchEvent(new CustomEvent(FOCUS_PANE_EVENT));
-    else return;
+    else if (e.key === "c") toggleHighlightedFolder();
+    else if (e.key === "x") {
+      const hit = highlightedSession();
+      if (hit) void requestKill(hit.session, sessionName(hit.session));
+    } else if (e.key === "r") {
+      const hit = highlightedSession();
+      if (hit) beginRename(hit.row, hit.session);
+    } else return;
     e.preventDefault();
   });
 

web/src/connections.ts

@@ -74,7 +74,7 @@ export class ConnectionPool {
    * Ensure `id` is live and shown inside `pane`, reusing its connection if one
    * exists (refreshing LRU recency). Does not touch other mounted panes.
    */
-  mount(id: Uuid, pane: HTMLElement, info?: AttachInfo): void {
+  mount(id: Uuid, pane: HTMLElement, info?: AttachInfo, opts?: { focus?: boolean }): void {
     let conn = this.cache.get(id);
     if (conn) {
       this.cache.delete(id);
@@ -93,9 +93,9 @@ export class ConnectionPool {
     }
     if (conn.el.parentElement !== pane) {
       pane.replaceChildren(conn.el);
-      conn.onMount();
+      conn.onMount(opts);
     } else if (!this.mounted.has(id)) {
-      conn.onMount();
+      conn.onMount(opts);
     }
     this.mounted.add(id);
     this.evict();
@@ -114,13 +114,17 @@ export class ConnectionPool {
   /**
    * Reconcile the visible set to exactly `entries` (in render order): park any
    * mounted session not listed, mount/refresh each listed one in its pane.
+   * `focus: false` shows the panes without taking the keyboard (preview).
    */
-  setVisible(entries: { id: Uuid; pane: HTMLElement; info?: AttachInfo }[]): void {
+  setVisible(
+    entries: { id: Uuid; pane: HTMLElement; info?: AttachInfo }[],
+    opts?: { focus?: boolean },
+  ): void {
     const next = new Set(entries.map((e) => e.id));
     for (const id of [...this.mounted]) {
       if (!next.has(id)) this.unmount(id);
     }
-    for (const e of entries) this.mount(e.id, e.pane, e.info);
+    for (const e of entries) this.mount(e.id, e.pane, e.info, opts);
   }
 
   /** Also tells the daemon which pane is focused (docs/SIDEBAR.md's

web/src/help.ts

@@ -31,6 +31,9 @@ export const SHORTCUTS: ShortcutGroup[] = [
       { keys: ["j"], description: "Highlight the next session" },
       { keys: ["k"], description: "Highlight the previous session" },
       { keys: ["Enter"], description: "Open the highlighted session, or expand a folder" },
+      { keys: ["c"], description: "Collapse or expand the highlighted folder" },
+      { keys: ["r"], description: "Rename the highlighted session" },
+      { keys: ["x"], description: "Kill the highlighted session" },
       { keys: ["Tab"], description: "Send the keyboard to the pane" },
       { keys: ["Esc"], description: "Hand the keyboard back to the session list, from the pane" },
     ],

web/src/main.ts

@@ -104,9 +104,26 @@ shellEl.dataset.route = "catalog";
 // after the welcome renderer). The hook only fires on user interaction, by which
 // point `mv` is assigned.
 let mv: MultiView;
+
+// Preview timing (docs/PREVIEW.md): a keyboard step shows almost at once; a
+// mouse sweep waits long enough that rows crossed on the way somewhere else
+// never flash up. The same delay applies to clearing, so the gap between two
+// rows doesn't blink the previous view back.
+const PREVIEW_DELAY = { key: 60, hover: 150 } as const;
+let previewTimer: number | undefined;
+function schedulePreview(sessionId: string | null, source: "key" | "hover"): void {
+  clearTimeout(previewTimer);
+  previewTimer = window.setTimeout(() => {
+    ensureMultiView();
+    mv?.previewSession(sessionId);
+    updateShellRoute();
+  }, PREVIEW_DELAY[source]);
+}
+
 const sidebar = renderSidebar(store, {
   onLeafDrag: (id, e) => mv?.beginSessionDrag(id, e),
   link,
+  onPreview: schedulePreview,
 });
 const content = document.createElement("section");
 content.className = "content";
@@ -184,7 +201,9 @@ function infoFor(sessionId: string) {
 
 /** Drive the shell's responsive layout off whether any pane is open. */
 function updateShellRoute(): void {
-  const hasPanes = openSessions(layout).size > 0;
+  // A preview is not an open pane (docs/PREVIEW.md), so the mobile shell does
+  // not swap the list out for a terminal on the way past a row.
+  const hasPanes = (mv?.openSessions() ?? openSessions(layout)).size > 0;
   shellEl.dataset.route = hasPanes ? "attach" : "catalog";
 }
 
@@ -244,11 +263,18 @@ mv = createMultiView({
   onLayoutChange: scheduleSave,
 });
 let contentShowsMv = false;
-function showMultiView(): void {
+/** Put the multi-view in the content pane, without rendering it. A preview
+ *  renders once, on its own terms (`docs/PREVIEW.md`); a render here would
+ *  first re-mount the pane it is about to replace, and that mount takes the
+ *  keyboard away from the list being walked. */
+function ensureMultiView(): void {
   if (!contentShowsMv) {
     content.replaceChildren(mv.el);
     contentShowsMv = true;
   }
+}
+function showMultiView(): void {
+  ensureMultiView();
   mv.render();
   updateShellRoute();
 }
@@ -375,7 +401,13 @@ window.addEventListener("keydown", (e) => {
 
 // Tab in the session list hands the keyboard to the pane; Escape in the pane
 // hands it back — the pair the TUI gets from `tab` and `esc`.
-document.addEventListener(FOCUS_PANE_EVENT, () => pool.focusTerminal());
+document.addEventListener(FOCUS_PANE_EVENT, () => {
+  // Handing the keyboard to a previewed pane is choosing it: the preview stops
+  // being provisional rather than snapping back the moment the list blurs.
+  const previewed = mv.previewedSessionId();
+  if (previewed) focusSession(previewed);
+  pool.focusTerminal();
+});
 document.addEventListener(FOCUS_SIDEBAR_EVENT, () => {
   drawer.open(); // no-op on desktop; on mobile the list is off-canvas
   sidebar.focusList();
@@ -386,6 +418,7 @@ document.addEventListener(FOCUS_SIDEBAR_EVENT, () => {
  *  on `hashchange` alone — a split or drag can move focus without the hash
  *  changing, leaving a click back onto that session's row silently ignored. */
 function focusSession(sessionId: string): void {
+  clearTimeout(previewTimer);
   showMultiView();
   mv.openSession(sessionId);
   updateShellRoute();

web/src/multiview.ts

@@ -11,6 +11,7 @@ import {
   addTab,
   closeTab,
   findLeaf,
+  freshId,
   type Dir,
   type Layout,
   leaves,
@@ -19,6 +20,7 @@ import {
   openSessions,
   removeLeaf,
   splitLeaf,
+  type Tab,
 } from "./layout.js";
 import type { AttachInfo } from "./attach.js";
 
@@ -57,11 +59,22 @@ export interface MultiView {
   /** Rebuild the view from the current layout. */
   render: () => void;
   focusedSessionId: () => string | null;
+  /** Show `sessionId` in the content pane without committing to it
+   *  (docs/PREVIEW.md): the tab holding it is brought forward, or a throwaway
+   *  tab carries it. `null` puts back whatever was on screen before. */
+  previewSession: (sessionId: string | null) => void;
+  /** The session being previewed, or null. */
+  previewedSessionId: () => string | null;
 }
 
 export function createMultiView(opts: MultiViewOptions): MultiView {
   const { pool, layout, sidebar, infoFor, onDisconnect, renderWelcome } = opts;
-  const persist = () => opts.onLayoutChange?.();
+  // A preview must never reach localStorage: it is undone the moment the
+  // highlight moves on, and the layout it would save is not the operator's.
+  const persist = () => {
+    if (preview) return;
+    opts.onLayoutChange?.();
+  };
 
   const el = document.createElement("div");
   el.className = "mv";
@@ -83,8 +96,119 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     render();
   });
 
+  // ---- preview (docs/PREVIEW.md) ----
+  // A preview is a temporary *view* change, never a layout change: it records
+  // exactly what it moved so ending it puts the layout back byte for byte, and
+  // it neither persists nor reports pane focus to the daemon while it is up.
+  interface Preview {
+    sessionId: string;
+    /** Tab that was active before the preview. */
+    prevActiveTabId: string;
+    /** Focused leaf to restore, when the preview stole focus in a real tab. */
+    prevFocus: { tabId: string; leafId: string | null } | null;
+    /** Throwaway tab built for a session that no pane was showing. */
+    tempTabId: string | null;
+  }
+  let preview: Preview | null = null;
+  /// Set while a preview-driven render runs: ending a preview re-mounts the
+  /// pane that was there, and that mount must not pull the keyboard out of the
+  /// sidebar the operator is still walking.
+  let quietRender = false;
+
+  /** Render without any mount taking the keyboard. */
+  function renderQuietly(): void {
+    quietRender = true;
+    try {
+      render();
+    } finally {
+      quietRender = false;
+    }
+  }
+
+  const isTempTab = (tabId: string) => preview?.tempTabId === tabId;
+
+  /** Undo the preview's moves. Mutates only — the caller renders. */
+  function restorePreview(): boolean {
+    const p = preview;
+    if (!p) return false;
+    preview = null;
+    if (p.tempTabId) {
+      const i = layout.tabs.findIndex((t) => t.id === p.tempTabId);
+      if (i >= 0) layout.tabs.splice(i, 1);
+    } else if (p.prevFocus) {
+      const t = layout.tabs.find((x) => x.id === p.prevFocus!.tabId);
+      if (t) t.focusedLeafId = p.prevFocus.leafId;
+    }
+    layout.activeTabId = layout.tabs.some((t) => t.id === p.prevActiveTabId)
+      ? p.prevActiveTabId
+      : layout.tabs[0].id;
+    return true;
+  }
+
+  /** Keep what the preview is showing: its throwaway tab becomes a real one,
+   *  and the focus it moved stands. Returns true when `sessionId` was the
+   *  session being previewed; any other preview is simply discarded. */
+  function settlePreview(sessionId: string): boolean {
+    const p = preview;
+    if (!p) return false;
+    if (p.sessionId !== sessionId) {
+      restorePreview();
+      return false;
+    }
+    preview = null;
+    if (p.tempTabId) {
+      const t = layout.tabs.find((x) => x.id === p.tempTabId);
+      if (t) t.title = `Tab ${layout.tabs.length}`;
+    }
+    return true;
+  }
+
+  function previewSession(sessionId: string | null): void {
+    if (preview?.sessionId === sessionId) return;
+    if (sessionId === null) {
+      if (restorePreview()) renderQuietly();
+      return;
+    }
+    restorePreview();
+    const prevActiveTabId = layout.activeTabId;
+    const loc = locateSession(layout, sessionId);
+    if (loc) {
+      preview = {
+        sessionId,
+        prevActiveTabId,
+        prevFocus: { tabId: loc.tab.id, leafId: loc.tab.focusedLeafId },
+        tempTabId: null,
+      };
+      layout.activeTabId = loc.tab.id;
+      loc.tab.focusedLeafId = loc.leaf.id;
+    } else {
+      // Not on screen anywhere: a throwaway tab carries it, so no existing
+      // pane is evicted and nothing the operator arranged moves.
+      const tab: Tab = { id: freshId("t"), title: "Preview", root: null, focusedLeafId: null };
+      tab.focusedLeafId = splitLeaf(tab, "", sessionId, "row", true);
+      layout.tabs.push(tab);
+      layout.activeTabId = tab.id;
+      preview = { sessionId, prevActiveTabId, prevFocus: null, tempTabId: tab.id };
+    }
+    renderQuietly();
+  }
+
+  /** Any user action that changes the layout for real ends the preview first,
+   *  so it never bakes a throwaway tab into the arrangement. */
+  function dropPreview(): void {
+    restorePreview();
+  }
+
   // ---- highlight helpers ----
 
+  /** Sessions the operator actually has open — the preview's throwaway tab
+   *  does not count, or hovering the sidebar would flip the shell's route. */
+  function realOpenSessions(): Set<string> {
+    const out = openSessions(layout);
+    if (preview?.tempTabId) out.delete(preview.sessionId);
+    return out;
+  }
+
   function focusedLeafSession(): string | null {
     const tab = activeTab(layout);
     const fl = tab.focusedLeafId ? findLeaf(tab.root, tab.focusedLeafId) : null;
@@ -98,9 +222,11 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     for (const pane of canvas.querySelectorAll<HTMLElement>(".mv-pane")) {
       pane.classList.toggle("is-focused", pane.dataset.leafId === tab.focusedLeafId);
     }
-    pool.setFocus(focusedLeafSession());
+    // A preview is not a focused pane: reporting it would clear the session's
+    // `AwaitingInput` glow just because the operator scrolled past it.
+    if (!preview) pool.setFocus(focusedLeafSession());
     sidebar.setHighlights({
-      open: openSessions(layout),
+      open: realOpenSessions(),
       focused: focusedLeafSession(),
     });
   }
@@ -168,6 +294,11 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     pane.addEventListener(
       "pointerdown",
       (e) => {
+        // Clicking into a previewed pane is the operator choosing it.
+        if (preview && leaf.sessionId === preview.sessionId) {
+          settlePreview(leaf.sessionId);
+          persist();
+        }
         focusLeaf(leaf.id);
         // Drag a pane by its title bar (desktop only — no docking on mobile).
         if (isMobile) return;
@@ -252,24 +383,31 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     for (const t of layout.tabs) {
       const tab = document.createElement("button");
       tab.type = "button";
-      tab.className = "mv-tab" + (t.id === layout.activeTabId ? " is-active" : "");
+      tab.className =
+        "mv-tab" +
+        (t.id === layout.activeTabId ? " is-active" : "") +
+        (isTempTab(t.id) ? " is-preview" : "");
       const n = leaves(t.root).filter((l) => l.sessionId).length;
       const label = document.createElement("span");
       label.textContent = t.title + (n ? ` · ${n}` : "");
       tab.appendChild(label);
       tab.addEventListener("click", () => {
+        if (isTempTab(t.id)) return; // a preview tab is not a place to go
+        dropPreview();
         if (layout.activeTabId !== t.id) {
           layout.activeTabId = t.id;
-          render();
         }
+        render();
       });
-      if (layout.tabs.length > 1) {
+      // A preview tab is not the operator's to close: it goes on its own.
+      if (layout.tabs.length > 1 && !isTempTab(t.id)) {
         const close = document.createElement("span");
         close.className = "mv-tab-close";
         close.textContent = "✕";
         close.title = "Close tab";
         close.addEventListener("click", (e) => {
           e.stopPropagation();
+          dropPreview();
           closeTab(layout, t.id);
           render();
         });
@@ -283,6 +421,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     add.textContent = "+";
     add.title = "New tab";
     add.addEventListener("click", () => {
+      dropPreview();
       addTab(layout);
       render();
     });
@@ -292,12 +431,14 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
   // ---- mutations ----
 
   function splitFocused(leafId: string, dir: Dir): void {
+    dropPreview();
     const tab = activeTab(layout);
     tab.focusedLeafId = splitLeaf(tab, leafId, null, dir, true);
     render();
   }
 
   function closePane(leafId: string): void {
+    dropPreview();
     const tab = activeTab(layout);
     removeLeaf(tab, leafId);
     if (tab.focusedLeafId === leafId) {
@@ -307,6 +448,12 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
   }
 
   function openSession(sessionId: string): void {
+    // Opening what is already previewed keeps exactly what is on screen: the
+    // throwaway tab becomes a real one instead of the session moving again.
+    if (settlePreview(sessionId)) {
+      render();
+      return;
+    }
     const loc = locateSession(layout, sessionId);
     if (loc) {
       layout.activeTabId = loc.tab.id;
@@ -337,6 +484,11 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
   /// (#67 review). `newTab` opens its own tab instead of halving whatever the
   /// operator was reading (#100) — what the Spawn button asks for.
   function openSessionBeside(sessionId: string, placement?: { newTab?: boolean }): void {
+    if (settlePreview(sessionId)) {
+      render();
+      opts.onOpenSessionsChange?.();
+      return;
+    }
     const loc = locateSession(layout, sessionId);
     if (loc) {
       layout.activeTabId = loc.tab.id;
@@ -384,13 +536,19 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     // A lone tab has nothing to switch between, so the bar is only a strip of
     // chrome; CSS hides it until a second tab exists. Spawning still opens its
     // own tab (#100), which brings the bar back with the "+" button on it.
-    el.classList.toggle("mv-solo-tab", layout.tabs.length === 1);
+    // The preview's throwaway tab must not make the bar appear and vanish as
+    // the highlight moves, so it does not count towards "one tab only".
+    el.classList.toggle(
+      "mv-solo-tab",
+      layout.tabs.filter((t) => !isTempTab(t.id)).length === 1,
+    );
+    el.classList.toggle("mv-previewing", preview !== null);
 
     if (!tab.root) {
       pool.setVisible([]);
-      pool.setFocus(null);
+      if (!preview) pool.setFocus(null);
       canvas.appendChild(renderWelcome());
-      sidebar.setHighlights({ open: openSessions(layout), focused: null });
+      sidebar.setHighlights({ open: realOpenSessions(), focused: null });
       persist();
       return;
     }
@@ -408,7 +566,12 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
 
     const hosts: { id: string; host: HTMLElement; info?: AttachInfo }[] = [];
     canvas.appendChild(buildTree(tab.root, hosts));
-    pool.setVisible(hosts.map((h) => ({ id: h.id, pane: h.host, info: h.info })));
+    // A preview keeps the keyboard in the sidebar: mounting must not focus the
+    // terminal, or j/k would stop walking the list after the first step.
+    pool.setVisible(
+      hosts.map((h) => ({ id: h.id, pane: h.host, info: h.info })),
+      { focus: !preview && !quietRender },
+    );
     syncFocus();
     persist();
   }
@@ -431,12 +594,14 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     // Only the focused leaf's session is live on mobile; the rest park.
     pool.setVisible(
       focused.sessionId ? [{ id: focused.sessionId, pane: pane.host, info: infoFor(focused.sessionId) }] : [],
+      { focus: !preview && !quietRender },
     );
-    pool.setFocus(focused.sessionId ?? null);
-    sidebar.setHighlights({ open: openSessions(layout), focused: focused.sessionId ?? null });
+    if (!preview) pool.setFocus(focused.sessionId ?? null);
+    sidebar.setHighlights({ open: realOpenSessions(), focused: focused.sessionId ?? null });
   }
 
   function closeSession(sessionId: string): void {
+    dropPreview();
     const loc = locateSession(layout, sessionId);
     if (!loc) return;
     removeLeaf(loc.tab, loc.leaf.id);
@@ -628,6 +793,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
   }
 
   function beginSessionDrag(sessionId: string, e: PointerEvent): void {
+    dropPreview();
     runDrag({ kind: "session", sessionId }, e, { deferred: true });
   }
 
@@ -636,9 +802,11 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     openSession,
     openSessionBeside,
     closeSession,
-    openSessions: () => openSessions(layout),
+    openSessions: realOpenSessions,
     beginSessionDrag,
     render,
     focusedSessionId: focusedLeafSession,
+    previewSession,
+    previewedSessionId: () => preview?.sessionId ?? null,
   };
 }

web/src/preview.test.ts

@@ -0,0 +1,218 @@
+// @vitest-environment happy-dom
+// Sidebar preview (docs/PREVIEW.md): highlighting a session shows it in the
+// content pane without changing the layout the operator built.
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import { addTab, emptyLayout, type Layout, splitLeaf } from "./layout.js";
+import { serializeLayout } from "./persistence.js";
+import { createMultiView, type MultiView } from "./multiview.js";
+import type { ConnectionPool } from "./connections.js";
+import type { SidebarHandle } from "./catalog.js";
+
+function stubFetch(): void {
+  vi.stubGlobal(
+    "fetch",
+    vi.fn(async (input: RequestInfo | URL) => {
+      const url = String(input);
+      const body = url.includes("/api/info")
+        ? { hostname: "h", version: "0", home_dir: "/home/user", presets: [] }
+        : undefined;
+      return new Response(JSON.stringify(body), { status: 200 });
+    }),
+  );
+}
+
+interface Spy {
+  pool: ConnectionPool;
+  visible: () => string[];
+  focusCalls: (string | null)[];
+  mountedWithFocus: (boolean | undefined)[];
+}
+
+function fakePool(): Spy {
+  let visible: string[] = [];
+  const focusCalls: (string | null)[] = [];
+  const mountedWithFocus: (boolean | undefined)[] = [];
+  const pool = {
+    setVisible(entries: { id: string }[], opts?: { focus?: boolean }) {
+      visible = entries.map((e) => e.id);
+      mountedWithFocus.push(opts?.focus);
+    },
+    setFocus(id: string | null) {
+      focusCalls.push(id);
+    },
+  } as unknown as ConnectionPool;
+  return { pool, visible: () => visible, focusCalls, mountedWithFocus };
+}
+
+function fakeSidebar(): SidebarHandle {
+  return {
+    el: document.createElement("aside"),
+    setActive() {},
+    setHighlights() {},
+    focusList() {},
+    hasListFocus() {
+      return false;
+    },
+    dispose() {},
+  };
+}
+
+function makeMv(layout: Layout, pool: ConnectionPool, onLayoutChange?: () => void): MultiView {
+  const mv = createMultiView({
+    pool,
+    layout,
+    sidebar: fakeSidebar(),
+    infoFor: () => ({ name: "sess", command: "bash" }),
+    onDisconnect: () => {},
+    renderWelcome: () => document.createElement("div"),
+    onLayoutChange,
+  });
+  document.body.appendChild(mv.el);
+  mv.render();
+  return mv;
+}
+
+/** Tab 1 holds `s1`; tab 2 holds `s2` and is the active one. */
+function twoTabs(): Layout {
+  const layout = emptyLayout();
+  const first = layout.tabs[0];
+  first.focusedLeafId = splitLeaf(first, "", "s1", "row", true);
+  const second = addTab(layout);
+  second.focusedLeafId = splitLeaf(second, "", "s2", "row", true);
+  layout.activeTabId = second.id;
+  return layout;
+}
+
+describe("sidebar preview", () => {
+  beforeEach(() => {
+    stubFetch();
+    document.body.replaceChildren();
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("brings a session's own tab forward, then puts the layout back", () => {
+    const layout = twoTabs();
+    const before = serializeLayout(layout);
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+
+    mv.previewSession("s1");
+    expect(spy.visible()).toEqual(["s1"]);
+    expect(mv.previewedSessionId()).toBe("s1");
+
+    mv.previewSession(null);
+    expect(spy.visible()).toEqual(["s2"]);
+    expect(mv.previewedSessionId()).toBeNull();
+    expect(serializeLayout(layout)).toEqual(before);
+  });
+
+  it("shows a session no pane holds in a throwaway tab that leaves no trace", () => {
+    const layout = twoTabs();
+    const before = serializeLayout(layout);
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+
+    mv.previewSession("s9");
+    expect(spy.visible()).toEqual(["s9"]);
+    expect(layout.tabs).toHaveLength(3);
+    // The throwaway tab is not something the operator has open.
+    expect(mv.openSessions().has("s9")).toBe(false);
+
+    mv.previewSession(null);
+    expect(layout.tabs).toHaveLength(2);
+    expect(serializeLayout(layout)).toEqual(before);
+  });
+
+  it("stepping between previews leaves only the newest one on screen", () => {
+    const layout = twoTabs();
+    const before = serializeLayout(layout);
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+
+    mv.previewSession("s9");
+    mv.previewSession("s1");
+    mv.previewSession("s8");
+    expect(spy.visible()).toEqual(["s8"]);
+    expect(layout.tabs).toHaveLength(3);
+
+    mv.previewSession(null);
+    expect(serializeLayout(layout)).toEqual(before);
+  });
+
+  it("opening what is previewed keeps it: the throwaway tab becomes real", () => {
+    const layout = twoTabs();
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+
+    mv.previewSession("s9");
+    mv.openSession("s9");
+
+    expect(mv.previewedSessionId()).toBeNull();
+    expect(layout.tabs).toHaveLength(3);
+    expect(mv.openSessions().has("s9")).toBe(true);
+    expect(spy.visible()).toEqual(["s9"]);
+    // Nothing the operator had open was evicted to make room.
+    expect(mv.openSessions().has("s2")).toBe(true);
+  });
+
+  it("never persists, and saves again once the preview is settled", () => {
+    const layout = twoTabs();
+    const spy = fakePool();
+    const saves = vi.fn();
+    const mv = makeMv(layout, spy.pool, saves);
+    saves.mockClear();
+
+    mv.previewSession("s9");
+    expect(saves).not.toHaveBeenCalled();
+
+    mv.previewSession(null);
+    expect(saves).toHaveBeenCalled();
+  });
+
+  it("never reports a previewed pane as focused to the daemon", () => {
+    const layout = twoTabs();
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+    spy.focusCalls.length = 0;
+
+    mv.previewSession("s1");
+    mv.previewSession("s9");
+    expect(spy.focusCalls).toEqual([]);
+
+    mv.previewSession(null);
+    expect(spy.focusCalls).toEqual(["s2"]);
+  });
+
+  it("mounts a preview without taking the keyboard", () => {
+    const layout = twoTabs();
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+    spy.mountedWithFocus.length = 0;
+
+    mv.previewSession("s1");
+    expect(spy.mountedWithFocus).toEqual([false]);
+
+    mv.openSession("s1");
+    expect(spy.mountedWithFocus.at(-1)).toBe(true);
+  });
+
+  it("keeps the tab bar hidden while a lone tab is previewing", () => {
+    const layout = emptyLayout();
+    const tab = layout.tabs[0];
+    tab.focusedLeafId = splitLeaf(tab, "", "s1", "row", true);
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+    expect(mv.el.classList.contains("mv-solo-tab")).toBe(true);
+
+    mv.previewSession("s9");
+    expect(mv.el.classList.contains("mv-solo-tab")).toBe(true);
+    expect(mv.el.classList.contains("mv-previewing")).toBe(true);
+
+    mv.previewSession(null);
+    expect(mv.el.classList.contains("mv-previewing")).toBe(false);
+  });
+});

web/src/sidebarNav.test.ts

@@ -39,8 +39,11 @@ const A = "aaaaaaaa-0000-0000-0000-00000000000a";
 const B = "aaaaaaaa-0000-0000-0000-00000000000b";
 
 /** Two sessions in sibling directories: two folder headers, two cards. */
-function twoFolders(): { el: HTMLElement; handle: ReturnType<typeof renderSidebar> } {
-  const handle = renderSidebar(storeWith([session(A, "/one"), session(B, "/two")]));
+function twoFolders(opts: Parameters<typeof renderSidebar>[1] = {}): {
+  el: HTMLElement;
+  handle: ReturnType<typeof renderSidebar>;
+} {
+  const handle = renderSidebar(storeWith([session(A, "/one"), session(B, "/two")]), opts);
   // Mounted: `focusList` moves real DOM focus, which needs a live document.
   document.body.replaceChildren(handle.el);
   return { el: handle.el, handle };
@@ -117,6 +120,43 @@ describe("sidebar keyboard navigation", () => {
     expect(asked).toBe(1);
   });
 
+  it("c collapses and expands the highlighted folder", () => {
+    const { el } = twoFolders();
+    const header = el.querySelectorAll<HTMLElement>(".tree-folder-header")[0];
+    header.click(); // collapse /one, so the walk stops on its header
+
+    press(tree(el), "j");
+    expect(highlighted(el)).toBe("folder:one");
+    press(tree(el), "c");
+    // Expanded: the card it was hiding is back in the walk.
+    expect(el.querySelector(`[data-nav-key="session:${A}"]`)).not.toBeNull();
+  });
+
+  it("r renames the highlighted session in place", () => {
+    const { el } = twoFolders();
+    press(tree(el), "j");
+    press(tree(el), "r");
+    const field = el.querySelector<HTMLInputElement>(".leaf-rename");
+    expect(field).not.toBeNull();
+    // Escape settles the rename, so the module-level "a row is being renamed"
+    // flag doesn't leak into the next test.
+    field!.dispatchEvent(new KeyboardEvent("keydown", { key: "Escape", bubbles: true }));
+  });
+
+  it("x kills the highlighted session", async () => {
+    const { el } = twoFolders();
+    const killed: string[] = [];
+    window.fetch = ((input: RequestInfo | URL, init?: RequestInit) => {
+      killed.push(`${init?.method ?? "GET"} ${String(input)}`);
+      return Promise.resolve(new Response(null, { status: 204 }));
+    }) as typeof window.fetch;
+
+    press(tree(el), "j");
+    press(tree(el), "x");
+    await Promise.resolve();
+    expect(killed.some((r) => r.startsWith("DELETE") && r.includes(A))).toBe(true);
+  });
+
   it("leaves the keyboard alone while a row is being renamed", () => {
     const { el } = twoFolders();
     press(tree(el), "j");
@@ -126,3 +166,43 @@ describe("sidebar keyboard navigation", () => {
     expect(highlighted(el)).toBe(`session:${A}`);
   });
 });
+
+describe("sidebar preview requests (docs/PREVIEW.md)", () => {
+  it("asks for a preview on every keyboard step, and for none once the list blurs", () => {
+    const seen: [string | null, string][] = [];
+    const { el } = twoFolders({ onPreview: (id, source) => seen.push([id, source]) });
+
+    press(tree(el), "j");
+    press(tree(el), "j");
+    expect(seen).toEqual([
+      [A, "key"],
+      [B, "key"],
+    ]);
+
+    tree(el).dispatchEvent(new FocusEvent("blur"));
+    expect(seen.at(-1)).toEqual([null, "key"]);
+  });
+
+  it("asks for a preview of the hovered card, and clears it on the way out", () => {
+    const seen: [string | null, string][] = [];
+    const { el } = twoFolders({ onPreview: (id, source) => seen.push([id, source]) });
+    const card = el.querySelectorAll<HTMLElement>(".tree-leaf")[0];
+
+    card.dispatchEvent(new PointerEvent("pointerover", { bubbles: true, pointerType: "mouse" }));
+    expect(seen).toEqual([[A, "hover"]]);
+    // Moving within the same card is not a new preview.
+    card.dispatchEvent(new PointerEvent("pointerover", { bubbles: true, pointerType: "mouse" }));
+    expect(seen).toHaveLength(1);
+
+    tree(el).dispatchEvent(new PointerEvent("pointerleave", { pointerType: "mouse" }));
+    expect(seen.at(-1)).toEqual([null, "hover"]);
+  });
+
+  it("ignores touch, where a tap already opens the session", () => {
+    const seen: (string | null)[] = [];
+    const { el } = twoFolders({ onPreview: (id) => seen.push(id) });
+    const card = el.querySelectorAll<HTMLElement>(".tree-leaf")[0];
+    card.dispatchEvent(new PointerEvent("pointerover", { bubbles: true, pointerType: "touch" }));
+    expect(seen).toEqual([]);
+  });
+});

web/src/styles.css

@@ -2023,6 +2023,51 @@ input.leaf-rename:focus {
   min-height: 0;
 }
 
+/* Mobile: the spawn form is the tallest dialog we have, so it takes the whole
+   screen and lays out as one flex column — header, scrolling body, actions
+   pinned to the bottom edge. Below the fold is where the Submit button used to
+   hide. */
+@media (max-width: 599px) {
+  .modal.spawn-modal {
+    height: 100dvh;
+    max-height: none;
+    border: 0;
+    border-radius: 0;
+  }
+
+  .spawn-modal .spawn-form {
+    flex: 1 1 auto;
+    padding-bottom: 0;
+  }
+
+  /* One scroll box, not four: the pickers' own caps stack scrollable regions
+     inside a screen that is already short. Let them grow and let the form
+     scroll. */
+  .spawn-modal .picker,
+  .spawn-modal .picker.projects,
+  .spawn-modal .picker.wt-choices {
+    max-height: none;
+    overflow-y: visible;
+  }
+
+  /* Rides the bottom of the scroll box: reachable at any scroll position, and
+     `margin-top: auto` keeps it at the screen's foot when the form is short. */
+  .spawn-modal .form-actions {
+    position: sticky;
+    bottom: 0;
+    margin-top: auto;
+    padding: var(--sp-2) 0 calc(var(--sp-2) + env(safe-area-inset-bottom, 0px));
+    background: var(--surface);
+    box-shadow: 0 -8px 12px -8px oklch(0 0 0 / 0.5);
+  }
+
+  .spawn-modal .form-actions .btn {
+    flex: 1 1 0;
+    justify-content: center;
+    min-height: 44px;
+  }
+}
+
 /* ---------- forms ---------- */
 
 .spawn-form {
@@ -2838,6 +2883,38 @@ input.leaf-rename:focus {
   color: var(--ink);
 }
 
+/* Preview (docs/PREVIEW.md): the content pane is showing a session the
+   operator only highlighted, so it reads as provisional — a dashed tab and a
+   dashed inset ring on the canvas — and reverts when the highlight moves. */
+.mv-tab.is-preview {
+  border-style: dashed;
+  font-style: italic;
+}
+.mv.mv-previewing .mv-canvas::before {
+  content: "preview";
+  position: absolute;
+  top: var(--sp-2);
+  right: var(--sp-3);
+  z-index: var(--z-sticky);
+  padding: 1px var(--sp-2);
+  border-radius: var(--r-1);
+  font: 500 var(--fs-11) / 1.5 var(--font-mono);
+  letter-spacing: 0.04em;
+  color: var(--accent-ink);
+  background: var(--accent);
+  opacity: 0.85;
+  pointer-events: none;
+}
+.mv.mv-previewing .mv-canvas::after {
+  content: "";
+  position: absolute;
+  inset: 0;
+  pointer-events: none;
+  border: 1px dashed var(--accent-ring);
+  border-radius: var(--r-1);
+  z-index: var(--z-sticky);
+}
+
 /* canvas + welcome */
 .mv-canvas {
   position: relative;

web/src/webglRenderer.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { MAX_CONTEXT_LOSSES, WebglPolicy } from "./webglRenderer.js";
+
+describe("WebglPolicy", () => {
+  it("is enabled when the browser supports WebGL2", () => {
+    expect(new WebglPolicy(() => true).enabled()).toBe(true);
+  });
+
+  it("is disabled when the browser has no WebGL2", () => {
+    expect(new WebglPolicy(() => false).enabled()).toBe(false);
+  });
+
+  it("probes only once, however often it is asked", () => {
+    const probe = vi.fn(() => true);
+    const policy = new WebglPolicy(probe);
+    policy.enabled();
+    policy.enabled();
+    policy.enabled();
+    expect(probe).toHaveBeenCalledTimes(1);
+  });
+
+  it("tolerates a loss below the budget — a backgrounded tab is ordinary", () => {
+    const policy = new WebglPolicy(() => true);
+    policy.noteContextLoss();
+    expect(policy.enabled()).toBe(true);
+  });
+
+  it("gives up for good once losses reach the budget", () => {
+    const policy = new WebglPolicy(() => true);
+    for (let i = 0; i < MAX_CONTEXT_LOSSES; i++) policy.noteContextLoss();
+    expect(policy.enabled()).toBe(false);
+    expect(policy.contextLosses()).toBe(MAX_CONTEXT_LOSSES);
+  });
+
+  it("stays disabled past the budget, and never bothers probing", () => {
+    const probe = vi.fn(() => true);
+    const policy = new WebglPolicy(probe);
+    for (let i = 0; i < MAX_CONTEXT_LOSSES + 3; i++) policy.noteContextLoss();
+    expect(policy.enabled()).toBe(false);
+    expect(probe).not.toHaveBeenCalled();
+  });
+
+  it("treats a throwing probe as unsupported", () => {
+    const policy = new WebglPolicy(() => {
+      throw new Error("no gl for you");
+    });
+    expect(() => policy.enabled()).not.toThrow();
+    expect(policy.enabled()).toBe(false);
+  });
+
+  it("reports no WebGL2 under the test DOM, which has no GPU", () => {
+    expect(new WebglPolicy().enabled()).toBe(false);
+  });
+});

web/src/webglRenderer.ts

@@ -0,0 +1,73 @@
+// WebGL renderer policy for attached terminals. Split out of attach.ts so it
+// can be tested without a DOM or an xterm instance (same reasoning as
+// reconnect.ts).
+//
+// xterm's default renderer builds DOM nodes per cell, which is CPU-bound —
+// exactly the wrong bottleneck on a phone, where a full-screen repaint from an
+// agent dumping output visibly stutters. The WebGL addon moves glyph drawing to
+// a GPU texture atlas instead.
+//
+// The cost is that a WebGL context is a scarce, revocable resource:
+//
+//   * Browsers cap live contexts (~16 on desktop Chrome, fewer on Android) and
+//     silently kill the oldest past the cap. `ConnectionPool` keeps MAX_PARKED
+//     parked connections alive on top of the mounted ones, so handing every
+//     connection its own context would blow that budget on its own and bring a
+//     parked terminal back blank.
+//   * Android drops contexts as routine housekeeping — backgrounding the tab,
+//     locking the screen, memory pressure.
+//
+// So the context follows the *view*, not the connection: acquired on mount,
+// released on park. Losses count against a shared budget, because a device that
+// keeps dropping contexts is telling us it cannot sustain one.
+
+/** Context losses tolerated before the page gives up and stays on the DOM
+ *  renderer. One loss is ordinary (a tab switch); repeated losses mean the
+ *  device is thrashing, and re-acquiring only makes it worse. */
+export const MAX_CONTEXT_LOSSES = 2;
+
+/** Can this browser hand out a WebGL2 context at all? */
+export function probeWebgl2(): boolean {
+  return document.createElement("canvas").getContext("webgl2") !== null;
+}
+
+/**
+ * Whether an attached terminal should try the WebGL renderer right now.
+ *
+ * Shared by every connection on the page: the context budget and the "this
+ * device cannot cope" signal are both global, not per-terminal.
+ */
+export class WebglPolicy {
+  private supported: boolean | null = null;
+  private losses = 0;
+
+  constructor(private readonly probe: () => boolean = probeWebgl2) {}
+
+  /** True while WebGL is both available and still worth attempting. The probe
+   *  runs at most once — support does not change under us, and creating
+   *  throwaway contexts to re-ask would itself consume the budget. */
+  enabled(): boolean {
+    if (this.losses >= MAX_CONTEXT_LOSSES) return false;
+    if (this.supported === null) {
+      try {
+        this.supported = this.probe();
+      } catch {
+        // A browser that throws rather than returning null has no usable
+        // context either way.
+        this.supported = false;
+      }
+    }
+    return this.supported;
+  }
+
+  /** Record a lost context. Past the budget, `enabled()` stays false for the
+   *  life of the page. */
+  noteContextLoss(): void {
+    this.losses += 1;
+  }
+
+  /** Losses recorded so far — for the debug HUD and tests. */
+  contextLosses(): number {
+    return this.losses;
+  }
+}