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/buttons/segmented.rs

@@ -110,8 +110,9 @@ impl RenderOnce for SegmentedControl {
              }| {
                 div()
                     .absolute()
-                    .top_0p5()
-                    .bottom_0p5()
+                    .top_0()
+                    .bottom_0()
+                    .debug_selector(|| "segment-thumb".into())
                     .rounded(theme.radius(Radius::Md))
                     .bg(colors.surface)
                     .border_1()
@@ -135,13 +136,13 @@ impl RenderOnce for SegmentedControl {
                 let on = ix == chosen;
                 let fg = if on { colors.fg } else { colors.fg_muted };
                 let (focus, reveal) = stops.remove(0);
-                let named = value.clone();
+                let (named, labeled) = (value.clone(), value.clone());
                 let (change, measure) = (self.on_change.clone(), state.clone());
                 let segment = div()
                     .id(("segment", ix))
                     .relative()
                     .flex()
-                    .flex_1()
+                    .flex_auto()
                     .min_w_0()
                     .items_center()
                     .justify_center()
@@ -176,15 +177,20 @@ impl RenderOnce for SegmentedControl {
                         segment.child(Icon::new(icon).size(icon_size).color(fg))
                     })
                     .when(!label.is_empty(), |segment| {
-                        segment.child(div().min_w_0().child(Ellipsis::new(label)))
-                    })
-                    .child(measure_item(measure, ix, Axis::Horizontal));
+                        segment.child(
+                            div()
+                                .min_w_0()
+                                .debug_selector(move || format!("segment-label {labeled}"))
+                                .child(Ellipsis::new(label)),
+                        )
+                    });
                 div()
                     .relative()
                     .flex()
-                    .flex_1()
+                    .flex_auto()
                     .min_w_0()
                     .child(segment)
+                    .child(measure_item(measure, ix, Axis::Horizontal))
                     .child(reveal)
             });
         let control = div()