Navigation: a wide tab strip scrolls and shows its chosen tab

A horizontal strip with more tabs than room clipped the rest out
of reach (ElyTable's sheets, Gitea #8). The strip now scrolls
sideways, the wheel included, and scrolls the chosen tab fully
into view whenever the choice changes; a manual scroll stays
until then. The tabs may shrink below their natural width.

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

src/navigation/tabs.rs

@@ -2,8 +2,8 @@ use std::rc::Rc;
 
 use gpui::{
     Animation, AnimationExt, AnyElement, App, ElementId, FontWeight, InteractiveElement,
-    IntoElement, MouseButton, ParentElement, Pixels, Point, RenderOnce, Role, SharedString,
+    IntoElement, MouseButton, ParentElement, Pixels, Point, RenderOnce, Role, ScrollHandle,
-    StatefulInteractiveElement, Styled, Window, div, prelude::*,
+    SharedString, StatefulInteractiveElement, Styled, Window, canvas, div, prelude::*,
 };
 
 use crate::{
@@ -108,6 +108,20 @@ impl RenderOnce for Tabs {
             window,
             cx,
         );
+        // A wide strip scrolls; the chosen tab scrolls in once.
+        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 focus = tab_stop((self.id.clone(), "focus").into(), true, window, cx);
         let focused = focus.is_focused(window);
         let tabs = Rc::new(self.tabs);
@@ -236,6 +250,9 @@ impl RenderOnce for Tabs {
             .relative()
             .flex()
             .when(axis == Axis::Vertical, |strip| strip.flex_col())
+            .when(axis == Axis::Horizontal, |strip| {
+                strip.min_w_0().overflow_x_scroll().track_scroll(&scroll)
+            })
             .gap_1()
             .border_color(colors.border)
             .map(|strip| match placement {
@@ -270,10 +287,27 @@ impl RenderOnce for Tabs {
                     key_pick(to, window, cx);
                 }
             })
+            // Items first: a child's index is its tab's.
+            .children(items)
             .child(measure_width(strip_width))
             .child(measure_origin(state, axis))
             .children(indicator)
-            .children(items);
+            .children(follow.map(|(followed, selected, scroll)| {
+                canvas(
+                    move |_, window, cx| {
+                        if scroll.bounds().size.width <= Pixels::ZERO {
+                            return;
+                        }
+                        followed.update(cx, |followed, _| *followed = Some(selected));
+                        if let Some(ix) = chosen {
+                            scroll.scroll_to_item(ix);
+                            window.request_animation_frame();
+                        }
+                    },
+                    |_, _, _, _| {},
+                )
+                .absolute()
+            }));
         let panel = self.panel.map(|panel| {
             div().flex_1().min_w_0().child(panel).with_animation(
                 ("tab-panel", turn),
@@ -282,7 +316,7 @@ impl RenderOnce for Tabs {
                 move |panel, t| if turn == 0 { panel } else { panel.opacity(t) },
             )
         });
-        let frame = div().id(self.id).flex().gap_4();
+        let frame = div().id(self.id).flex().min_w_0().gap_4();
         match placement {
             TabPlacement::Top => frame.flex_col().child(strip).children(panel),
             TabPlacement::Bottom => frame.flex_col().children(panel).child(strip),

src/navigation/tests/tabs.rs

@@ -180,3 +180,65 @@ fn tabs_whose_choice_closed_mark_none_and_arrows_start_at_the_ends(cx: &mut Test
     cx.simulate_keystrokes("left");
     assert_eq!(view.read_with(cx, |view, _| view.chosen.clone()), "c");
 }
+
+struct Sheets {
+    chosen: SharedString,
+    clicked: Option<SharedString>,
+}
+
+impl Render for Sheets {
+    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let view = cx.entity();
+        let tabs = (0..30).map(|ix| Choice::new(format!("s{ix}"), format!("Sheet {ix}")));
+        div()
+            .w(px(300.0))
+            .child(
+                Tabs::new("sheets", tabs, self.chosen.clone()).on_change(move |value, _, cx| {
+                    view.update(cx, |view, _| view.clicked = Some(value.clone()))
+                }),
+            )
+    }
+}
+
+/// The tab number a click at `x` picks.
+fn picked_at(x: f32, view: &gpui::Entity<Sheets>, cx: &mut VisualTestContext) -> usize {
+    view.update(cx, |view, _| view.clicked = None);
+    click_at(x, cx);
+    let clicked = view
+        .read_with(cx, |view, _| view.clicked.clone())
+        .expect("a tab is picked");
+    clicked[1..].parse().expect("a tab number")
+}
+
+#[gpui::test]
+fn a_wide_strip_scrolls_its_chosen_tab_in_and_the_wheel_reaches_the_rest(cx: &mut TestAppContext) {
+    setup(cx);
+    let (view, cx) = cx.add_window_view(|_, _| Sheets {
+        chosen: "s29".into(),
+        clicked: None,
+    });
+    settle(cx);
+    assert!(
+        picked_at(10.0, &view, cx) > 20,
+        "the strip starts at its last tab"
+    );
+    cx.simulate_event(ScrollWheelEvent {
+        position: point(px(60.0), px(16.0)),
+        delta: ScrollDelta::Pixels(point(px(0.0), px(5000.0))),
+        modifiers: Modifiers::none(),
+        touch_phase: TouchPhase::Moved,
+    });
+    view.update(cx, |_, cx| cx.notify());
+    settle(cx);
+    assert_eq!(
+        picked_at(10.0, &view, cx),
+        0,
+        "the wheel scrolls back to the first tab"
+    );
+    view.update(cx, |view, cx| {
+        view.chosen = "s25".into();
+        cx.notify();
+    });
+    settle(cx);
+    assert!(picked_at(10.0, &view, cx) > 15, "a new choice scrolls in");
+}