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.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/documents/reading.rs

@@ -208,27 +208,23 @@ impl RenderOnce for ReadingProgress {
         let theme = cx.theme();
         let (track, bar) = (theme.colors.border, theme.colors.accent);
         let scroll = self.scroll;
-        div()
-            .w_full()
-            .h(theme.progress_thickness())
-            .bg(track)
-            .child(
-                canvas(
-                    |_, _, _| {},
-                    move |bounds, _, window, _| {
-                        let share = read_share(
-                            f32::from(scroll.offset().y),
-                            f32::from(scroll.max_offset().height),
-                        );
-                        let filled = Bounds::new(
-                            bounds.origin,
-                            size(bounds.size.width * share, bounds.size.height),
-                        );
-                        window.paint_quad(fill(filled, bar));
-                    },
-                )
-                .size_full(),
+        div().h(theme.progress_thickness()).bg(track).child(
+            canvas(
+                |_, _, _| {},
+                move |bounds, _, window, _| {
+                    let share = read_share(
+                        f32::from(scroll.offset().y),
+                        f32::from(scroll.max_offset().height),
+                    );
+                    let filled = Bounds::new(
+                        bounds.origin,
+                        size(bounds.size.width * share, bounds.size.height),
+                    );
+                    window.paint_quad(fill(filled, bar));
+                },
             )
+            .size_full(),
+        )
     }
 }