Capture · Roots: the roots round 2 missed

A sweep that read every render found 28 roots still on w_full,
reached by 64 components: the field button under Select and every
picker, the checked and mention fields, the slider track,
NumberInput, Steps, the state views, the agent previews, and every
chart and finance frame. All drop it. LivePreview's size_full becomes
h_full, since its card no longer fills.

Every host of those roots was listed and read, 231 in all. Five
tests that mounted a picker as the window root sit in a size_full
box, the index manager gives its method select a flex_1 box, and the
notification center and the gallery's state frames stack their state
views in a column so they stretch.

Each dropped root carries a debug selector, and six chapter tests
mount 43 components in a padded card whose column is measured by
content, through layout::tests::narrow_width: each root must take the
full 240px. With w_full back on the roots, all six fail.

AGENTS.md names the roots that keep w_full and why: picture frames
that keep a ratio, and rows that hand a text field its share.

src/layout/mod.rs

@@ -12,6 +12,8 @@ mod sidebar;
 mod split;
 mod stack;
 mod surface;
+#[cfg(all(test, feature = "test-support"))]
+pub(crate) mod tests;
 mod viewport;
 mod workspace;
 

src/layout/tests.rs

@@ -0,0 +1,34 @@
+use gpui::{
+    AnyElement, App, Context, IntoElement, ParentElement, Pixels, Render, Styled, TestAppContext,
+    Window, div, px,
+};
+
+type Build = Box<dyn Fn(&mut Window, &mut App) -> AnyElement>;
+
+/// One element in a column that a padded block measures by content.
+struct Narrow(Build);
+
+impl Render for Narrow {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        div().w(px(280.0)).flex().flex_col().child(
+            div().p_5().child(
+                div()
+                    .flex()
+                    .flex_col()
+                    .child((self.0)(window, cx))
+                    .child("A line long enough to wrap in a narrow card, so the column fills it."),
+            ),
+        )
+    }
+}
+
+/// The width of `selector` in that column; a root that fills it takes all 240px.
+pub(crate) fn narrow_width(
+    cx: &mut TestAppContext,
+    selector: &'static str,
+    build: impl Fn(&mut Window, &mut App) -> AnyElement + 'static,
+) -> Pixels {
+    let (_, cx) = cx.add_window_view(|_, _| Narrow(Box::new(build)));
+    cx.run_until_parked();
+    cx.debug_bounds(selector).expect(selector).size.width
+}