Theme · Editing: segments sized by their labels, a thumb that covers its segment

A segmented control's segments now take their labels' widths first and
share the spare room, so a label ends in an ellipsis only when the whole
strip lacks room. The thumb's insets meet the strip's padding again, and
each segment is measured from its borderless wrapper, so the thumb covers
the chosen segment exactly. Tests hold Comfortable whole at 280px and the
thumb to the chosen segment's bounds.

src/settings/tests/theme.rs

@@ -118,3 +118,36 @@ fn the_density_strip_fits_a_narrow_box_with_larger_text(cx: &mut TestAppContext)
         .expect("the last segment");
     assert!(last.right() <= frame.right(), "{last:?} inside {frame:?}");
 }
+
+/// At 280px and the base size the strip has room, so each label keeps its whole width: eleven letters to seven.
+#[gpui::test]
+fn a_label_gives_way_only_when_the_strip_lacks_room(cx: &mut TestAppContext) {
+    let (_, cx) = desk(narrow, cx);
+    settle(cx);
+    let compact = cx
+        .debug_bounds("segment-label Compact")
+        .expect("Compact's label");
+    let comfortable = cx
+        .debug_bounds("segment-label Comfortable")
+        .expect("Comfortable's label");
+    let ratio = comfortable.size.width / compact.size.width;
+    assert!(
+        (ratio - 11.0 / 7.0).abs() < 0.01,
+        "{comfortable:?} to {compact:?}"
+    );
+}
+
+/// The thumb covers the chosen segment, top to bottom and side to side.
+#[gpui::test]
+fn the_thumb_covers_the_chosen_segment(cx: &mut TestAppContext) {
+    let (_, cx) = desk(editor, cx);
+    for _ in 0..3 {
+        std::thread::sleep(std::time::Duration::from_millis(2));
+        settle(cx);
+    }
+    let thumb = cx.debug_bounds("segment-thumb").expect("the thumb");
+    let chosen = cx
+        .debug_bounds("segment Standard")
+        .expect("the chosen segment");
+    assert_eq!(thumb, chosen);
+}