Panics · P05 Buttons: a segmented choice missing from its segments marks none; slide logs and shows no marker

src/buttons/segmented.rs

@@ -69,10 +69,7 @@ impl RenderOnce for SegmentedControl {
             .iter()
             .map(|(value, ..)| value.clone())
             .collect();
-        let chosen = values
-            .iter()
-            .position(|value| *value == self.selected)
-            .unwrap_or_else(|| panic!("segmented control has no segment {}", self.selected));
+        let chosen = values.iter().position(|value| *value == self.selected);
         let (state, marker) = slide(self.id.clone(), &values, &self.selected, window, cx);
         let scroll = window
             .use_keyed_state((self.id.clone(), "scroll"), cx, |_, _| ScrollHandle::new())
@@ -133,7 +130,7 @@ impl RenderOnce for SegmentedControl {
             .into_iter()
             .enumerate()
             .map(|(ix, (value, label, icon))| {
-                let on = ix == chosen;
+                let on = Some(ix) == chosen;
                 let fg = if on { colors.fg } else { colors.fg_muted };
                 let (focus, reveal) = stops.remove(0);
                 let (named, labeled) = (value.clone(), value.clone());

src/buttons/tests.rs

@@ -3,7 +3,7 @@ use gpui::{
     Render, Styled, TestAppContext, VisualTestContext, Window, div, point, px,
 };
 
-use super::{Button, ToggleButton, ToggleItem};
+use super::{Button, SegmentedControl, ToggleButton, ToggleItem};
 use crate::{
     primitives::{FocusNext, FocusScope},
     theme::Theme,
@@ -98,3 +98,31 @@ fn a_disabled_toggle_ignores_clicks(cx: &mut TestAppContext) {
     cx.simulate_click(point(px(4.0), px(4.0)), Modifiers::none());
     assert_eq!(view.read_with(cx, |toggles, _| toggles.presses), 1);
 }
+
+/// Segments whose chosen value left with a change of list.
+struct Stale;
+
+impl Render for Stale {
+    fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
+        div().w(px(300.0)).child(
+            SegmentedControl::new("views", "gone")
+                .segment("list", "List", None)
+                .segment("grid", "Grid", None),
+        )
+    }
+}
+
+#[gpui::test]
+fn a_choice_missing_from_its_segments_marks_none(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let (_, cx) = cx.add_window_view(|_, _| Stale);
+    for _ in 0..3 {
+        cx.run_until_parked();
+        cx.update(|window, _| window.refresh());
+    }
+    assert!(cx.debug_bounds("segment list").is_some());
+    assert!(
+        cx.debug_bounds("segment-thumb").is_none(),
+        "no thumb marks a choice"
+    );
+}

src/motion/slide.rs

@@ -46,7 +46,7 @@ pub(crate) struct Marker {
     pub generation: u64,
 }
 
-/// The slide over `values` with `selected` chosen; no marker until that item is measured.
+/// The slide over `values` with `selected` chosen; no marker until it is measured, or if it is not there.
 pub(crate) fn slide(
     id: impl Into<ElementId>,
     values: &[SharedString],
@@ -54,14 +54,15 @@ pub(crate) fn slide(
     window: &mut Window,
     cx: &mut App,
 ) -> (Entity<Slide>, Option<Marker>) {
-    let chosen = values
-        .iter()
-        .position(|value| value == selected)
-        .unwrap_or_else(|| panic!("no item {selected} to slide to"));
-    let state = window.use_keyed_state(id, cx, |_, _| Slide::default());
+    let id = id.into();
+    let state = window.use_keyed_state(id.clone(), cx, |_, _| Slide::default());
     if state.read(cx).spans.len() != values.len() {
         state.update(cx, |slide, _| slide.spans = vec![None; values.len()]);
     }
+    let Some(chosen) = values.iter().position(|value| value == selected) else {
+        log::error!("slide {id:?}: no item {selected} among {values:?}; nothing marked");
+        return (state, None);
+    };
     if state.read(cx).last.as_ref() != Some(selected) {
         state.update(cx, |slide, _| {
             let previous = slide

tasks/panics.md

@@ -19,7 +19,7 @@ Each item reads every site in its folders, fixes what the rules name, updates ca
 - [x] P02 Typography — `src/typography`
 - [x] P03 Layout — `src/layout`
 - [x] P04 Window & Shell — `src/shell`
-- [ ] P05 Buttons & Actions — `src/buttons`
+- [x] P05 Buttons & Actions — `src/buttons`
 - [ ] P06 Forms — `src/forms`
 - [ ] P07 Navigation — `src/navigation`
 - [ ] P08 Menus — `src/menus`
@@ -69,3 +69,4 @@ One line per item: what changed, what stayed and why, review rounds.
 - P02: `Link::new` takes its handler; `system_zone` logs once and reads as jiff's unknown zone, which `datetime` marks UTC; an oversized highlight query logs and marks nothing; a cut line's paint fault logs. Kept: caller contracts (speed, digits, step, keystroke, heading level, pattern) and the exponent parse. Review: 3 rounds.
 - P03: `PaneGroup` leaves a gone or last pane and logs (`split` returns `Option`); a stale split path sets nothing; `SplitPane` splits evenly when sizes lag or fail, seeds by the owner's raw sizes, and drops a drag over another pane count; masonry sorts with `total_cmp`. Kept: dock ids (panels never leave), one-value contracts, pane-id overflow, workspace JSON. Review: 2 rounds.
 - P04: no change. Kept: splash progress and column count (caller contracts), the objc class under `Once`, `Tray::pick` (items fixed at creation). Review: 1 round.
+- P05: a `SegmentedControl` whose choice left its segments marks none; the cause, `motion::slide`, logs and returns no marker (P11 inherits it). Kept: one-value contracts (group and quick-action counts, toggle label or icon, share payload). Review: 1 round.