maestro/docs/PREVIEW.md

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.

How a preview reads

A preview is provisional, so it never wears the accent — the accent is what says "this is the one you picked". It has a hue of its own, and every surface involved in the preview carries that same hue, so the pane and the row it came from read as one thing rather than two unrelated highlights.

Web client (--preview and friends in web/src/styles.css):

Surface	While previewing
The previewed pane's titlebar	Tinted in the preview hue, at full strength, in place of the focused pane's accent tint.
The canvas	A preview chip, top right, and a dashed inset ring.
A throwaway tab	Dashed border, italic label.
The sidebar row being previewed	Filled with the preview hue, over whatever open state the row already had — but the row of the session already showing in the pane keeps its accent fill, since previewing it changes nothing on screen.
A hovered sidebar row, and the keyboard cursor on a session	A lighter wash of the same hue, because both are what previews a row.

Rows say it with fill, not with an edge — see the tier ladder in docs/SIDEBAR.md, which previewing sits at the bottom of. The dashed edges belong to the pane: the canvas ring and the throwaway tab.

The hue is a palette token, not a fixed colour: a palette whose accent already sits on it moves it (--preview-h), so "previewing" and "picked" never collide.

The TUI marks a preview in text only — the title bar reads <name> · preview — and has no preview colour role yet.

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 is never delayed. Landing on a row — by pointer or by j/k — shows it on the next frame, and so does leaving every row. There is no settling period in front of the first preview, because that period would sit in front of the one preview the operator actually meant.

What is bounded is how often the pane may swap: at most once per 120 ms. A burst inside one window — j held down, or the pointer sweeping the length of the sidebar — shows the row it started on, then the row it ended on, and none of the rows in between. So a walk costs a couple of pane mounts rather than one per row, without the operator ever waiting on one.

The highlight itself is not bounded by any of this: the row lights up on the same frame as the input, whatever the pane behind it is doing.

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.