Navigation: the chosen tab follows a strip that changes width

The strip scrolled its chosen tab in once, at its first width; a
panel opening beside it then narrowed it and pushed the tab out
of view (ElyTable on a 30-sheet workbook). The strip now follows
its chosen tab whenever the choice or its width changes, checked
at paint with the width just laid out; a manual scroll stays
until either changes.

Co-Authored-By: Claude <noreply@anthropic.com>

src/navigation/tabs.rs

@@ -108,20 +108,16 @@ impl RenderOnce for Tabs {
             window,
             cx,
         );
-        // A wide strip scrolls; the chosen tab scrolls in once.
+        // The chosen tab scrolls in per choice and width.
         let scroll = window
             .use_keyed_state((self.id.clone(), "scroll"), cx, |_, _| ScrollHandle::new())
             .read(cx)
             .clone();
-        let followed =
-            window.use_keyed_state(
-                (self.id.clone(), "followed"),
-                cx,
-                |_, _| None::<SharedString>,
-            );
-        let follow = (axis == Axis::Horizontal
-            && followed.read(cx).as_ref() != Some(&self.selected))
-        .then(|| (followed, self.selected.clone(), scroll.clone()));
+        let followed = window.use_keyed_state((self.id.clone(), "followed"), cx, |_, _| {
+            None::<(SharedString, Pixels)>
+        });
+        let follow =
+            (axis == Axis::Horizontal).then(|| (followed, self.selected.clone(), scroll.clone()));
         let focus = tab_stop((self.id.clone(), "focus").into(), true, window, cx);
         let focused = focus.is_focused(window);
         let tabs = Rc::new(self.tabs);
@@ -295,10 +291,11 @@ impl RenderOnce for Tabs {
             .children(follow.map(|(followed, selected, scroll)| {
                 canvas(
                     move |_, window, cx| {
-                        if scroll.bounds().size.width <= Pixels::ZERO {
+                        let now = (selected, scroll.bounds().size.width);
+                        if now.1 <= Pixels::ZERO || followed.read(cx).as_ref() == Some(&now) {
                             return;
                         }
-                        followed.update(cx, |followed, _| *followed = Some(selected));
+                        followed.update(cx, |followed, _| *followed = Some(now));
                         if let Some(ix) = chosen {
                             scroll.scroll_to_item(ix);
                             window.request_animation_frame();

src/navigation/tests/tabs.rs

@@ -184,6 +184,7 @@ fn tabs_whose_choice_closed_mark_none_and_arrows_start_at_the_ends(cx: &mut Test
 struct Sheets {
     chosen: SharedString,
     clicked: Option<SharedString>,
+    width: f32,
 }
 
 impl Render for Sheets {
@@ -191,7 +192,7 @@ impl Render for Sheets {
         let view = cx.entity();
         let tabs = (0..30).map(|ix| Choice::new(format!("s{ix}"), format!("Sheet {ix}")));
         div()
-            .w(px(300.0))
+            .w(px(self.width))
             .child(
                 Tabs::new("sheets", tabs, self.chosen.clone()).on_change(move |value, _, cx| {
                     view.update(cx, |view, _| view.clicked = Some(value.clone()))
@@ -216,6 +217,7 @@ fn a_wide_strip_scrolls_its_chosen_tab_in_and_the_wheel_reaches_the_rest(cx: &mu
     let (view, cx) = cx.add_window_view(|_, _| Sheets {
         chosen: "s29".into(),
         clicked: None,
+        width: 300.0,
     });
     settle(cx);
     assert!(
@@ -242,3 +244,24 @@ fn a_wide_strip_scrolls_its_chosen_tab_in_and_the_wheel_reaches_the_rest(cx: &mu
     settle(cx);
     assert!(picked_at(10.0, &view, cx) > 15, "a new choice scrolls in");
 }
+
+#[gpui::test]
+fn a_strip_that_narrows_keeps_its_chosen_tab_in_view(cx: &mut TestAppContext) {
+    setup(cx);
+    let (view, cx) = cx.add_window_view(|_, _| Sheets {
+        chosen: "s29".into(),
+        clicked: None,
+        width: 700.0,
+    });
+    settle(cx);
+    view.update(cx, |view, cx| {
+        view.width = 250.0;
+        cx.notify();
+    });
+    settle(cx);
+    assert!(picked_at(10.0, &view, cx) > 25, "the strip shows its end");
+    view.update(cx, |view, _| view.clicked = None);
+    click_at(245.0, cx);
+    let clicked = view.read_with(cx, |view, _| view.clicked.clone());
+    assert_eq!(clicked, None, "the chosen tab ends the strip");
+}