diff --git a/crates/maestro-daemon/src/session.rs b/crates/maestro-daemon/src/session.rs
index c2229ee..4070b24 100644
--- a/crates/maestro-daemon/src/session.rs
+++ b/crates/maestro-daemon/src/session.rs
@@ -597,6 +597,9 @@ impl SessionManager {
                     status: SessionStatus::Running,
                     created_at: Utc::now(),
                     // Activity is unknown on adoption; the watcher fills it in.
+                    // #104: the restart also resets the activity clock to now —
+                    // accepted, since the daemon cannot know how long a
+                    // recovered session had already been sitting idle.
                     foreground: None,
                     activity: ActivityState::Idle,
                     last_activity_at: Utc::now(),
@@ -857,6 +860,12 @@ async fn run_activity_pass(inner: &Arc<Inner>) {
             {
                 None
             } else {
+                // #104: stamp only when the state itself changed — never on
+                // foreground/cwd-only diffs, and never every tick, or the
+                // sidebar's duration would reset continuously and read zero.
+                if rec.session.activity != activity {
+                    rec.session.activity_since = Utc::now();
+                }
                 rec.session.activity = activity;
                 rec.session.foreground = foreground;
                 rec.session.current_cwd = current_cwd;
diff --git a/crates/maestro-daemon/tests/session_decisions.rs b/crates/maestro-daemon/tests/session_decisions.rs
index 3b81634..dae0158 100644
--- a/crates/maestro-daemon/tests/session_decisions.rs
+++ b/crates/maestro-daemon/tests/session_decisions.rs
@@ -363,6 +363,9 @@ async fn activity_pass_emits_updated_only_when_something_changed() {
     // Still inside `activity::ACTIVE_WINDOW` (3s) since the spawn, so the
     // change here is `foreground`, not the activity state.
     assert_eq!(updated.activity, ActivityState::Active);
+    // A foreground/cwd-only change is not a state transition (#104): the
+    // sidebar's duration must keep counting from the original stamp.
+    assert_eq!(updated.activity_since, session.activity_since);
 
     // Drain anything already queued, then confirm a steady pane stays quiet.
     // Stays well inside the 3s window, so no Active -> Idle transition either.
@@ -374,6 +377,70 @@ async fn activity_pass_emits_updated_only_when_something_changed() {
     );
 }
 
+/// The sidebar's duration (#104) is time in the *current* state, so
+/// `activity_since` is stamped exactly when the decided state differs from
+/// the stored one — once per transition. Stamping it every tick would make
+/// the duration permanently read zero.
+#[tokio::test]
+async fn activity_transition_stamps_activity_since_once_per_transition() {
+    let fx = Fx::new();
+    let session = fx
+        .manager
+        .create(fx.request("sleep 300"))
+        .await
+        .expect("create");
+    fx.fake.set_pane_info(
+        session.id,
+        PaneInfo {
+            // Nothing reads from this pid: `awaiting_input` is deterministically
+            // false, so past `ACTIVE_WINDOW` the session decides as Idle.
+            pane_pid: i32::MAX,
+            command: "sleep".into(),
+            tty: "/dev/null".into(),
+            current_path: String::new(),
+        },
+    );
+    let mut events = fx.manager.events();
+    fx.manager.start_liveness_watcher(Duration::from_millis(25));
+
+    let idle = wait_for_activity(&fx.manager, session.id, ActivityState::Idle).await;
+    assert!(
+        idle.activity_since >= session.activity_since + chrono::Duration::seconds(2),
+        "the Active -> Idle transition must stamp a fresh activity_since \
+         (spawn stamp {:?}, transition stamp {:?})",
+        session.activity_since,
+        idle.activity_since
+    );
+
+    // Steady Idle across many more ticks must not re-stamp — no further
+    // SessionUpdated at all, or the sidebar clock would keep resetting.
+    while events.try_recv().is_ok() {}
+    tokio::time::sleep(Duration::from_millis(300)).await;
+    assert!(
+        matches!(events.try_recv(), Err(TryRecvError::Empty)),
+        "a steady state must not re-emit (and re-stamp) on every tick"
+    );
+}
+
+async fn wait_for_activity(
+    manager: &SessionManager,
+    id: uuid::Uuid,
+    want: ActivityState,
+) -> maestro_protocol::Session {
+    let deadline = tokio::time::Instant::now() + Duration::from_secs(6);
+    loop {
+        if tokio::time::Instant::now() >= deadline {
+            panic!("timed out waiting for {id} to become {want:?}");
+        }
+        if let Some(s) = manager.snapshot().await.into_iter().find(|s| s.id == id) {
+            if s.activity == want {
+                return s;
+            }
+        }
+        tokio::time::sleep(Duration::from_millis(20)).await;
+    }
+}
+
 async fn wait_for_updated(
     events: &mut tokio::sync::broadcast::Receiver<CatalogEvent>,
 ) -> maestro_protocol::Session {