Capture · Roots: roots drop w_full; hosts give the width

145 component roots drop w_full, the early returns included. Three
field rows keep it, Input, InputGroup and TagInput: stretched without
a width, taffy 0.9 laid the field's flex_1 share at no width in the
template picker at 280px, and the placeholder drew nothing. Container
keeps it, since mx_auto needs a width to fill.

Hosts that relied on a root's width now give it: MessageList boxes
each row at the list's width, the splash gives its bar the column's,
and test stages that mount a component as the window's root sit in a
size_full box. Tests hold a narrow row across the list and the splash
bar across its column; each goes red without its fix.

The gallery gains --narrow <px>, which lays each page in a padded
card at that width and shoots it without its script. The page
header wraps: the title takes the room and the theme switcher drops
below, where a long summary used to push it past the edge. A file
panel step waits for the panel to become key instead of a fixed
pause, which lost a race in a capture.

src/documents/blocks/tests.rs

@@ -12,12 +12,28 @@ fn open(
         forms::bind_keys(cx);
         crate::documents::bind_keys(cx);
     });
-    let (editor, cx) = cx.add_window_view(|window, cx| BlockEditor::new(blocks, window, cx));
+    let (full, cx) =
+        cx.add_window_view(|window, cx| Full(cx.new(|cx| BlockEditor::new(blocks, window, cx))));
+    let editor = full.read_with(cx, |full, _| full.0.clone());
     cx.update(|window, _| window.activate_window());
     settle(cx);
     (editor, cx)
 }
 
+/// The editor as a window's content, in the box that fills the window.
+struct Full(Entity<BlockEditor>);
+
+impl gpui::Render for Full {
+    fn render(
+        &mut self,
+        _: &mut gpui::Window,
+        _: &mut gpui::Context<Self>,
+    ) -> impl gpui::IntoElement {
+        use gpui::{ParentElement, Styled};
+        gpui::div().size_full().child(self.0.clone())
+    }
+}
+
 fn settle(cx: &mut VisualTestContext) {
     cx.run_until_parked();
     cx.update(|window, _| window.refresh());