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.

AGENTS.md

@@ -153,7 +153,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A row that holds text beside fixed parts lets the text give way: the text takes `flex_1().min_w_0()` and an `Ellipsis`; counts, icons, dates and chevrons take `flex_none`. A label that must stay whole wraps in that block instead, as `forms::Checkbox`'s does.
 - A mark over a text field reads the field at paint time, from a canvas laid over it (`collab::RemoteCursor`): by then `TextInput::bounds_for` holds this frame's layout, and a still page never renders again to catch up.
 - A callback field holds a named `type` alias; clippy's type_complexity rejects `Option<Rc<dyn Fn(..)>>`. A chapter shares one alias per shape.
-- A component that fills its container takes no width on its root: a block or a flex column stretches it. Percents fill badly in taffy 0.9. `w_full` in a flex column that a block measures by content keeps its content's width: taffy lays the column out once at unknown width and keeps that layout when the width comes out the same. `min_w_full` resolves against a block's outer width, padding and all (`forms::FormField`). In a flex row the host gives it `flex_1`; as a window's root it sits in a `size_full` box. A root centered under a cap keeps `w_full`, since `mx_auto` needs a width to fill (`layout::Container`); so does one that keeps a ratio, since taffy 0.9 follows a ratio from a set width, not a stretch (`layout::AspectRatio`). A row that hands a text field a `flex_1` share keeps `w_full` too: stretched without a width, taffy 0.9 can keep a pass that laid the share at no width, and the field draws nothing (`forms::Input`, `InputGroup`, `TagInput`).
+- A component that fills its container takes no width on its root: a block or a flex column stretches it. Percents fill badly in taffy 0.9. `w_full` in a flex column that a block measures by content keeps its content's width: taffy lays the column out once at unknown width and keeps that layout when the width comes out the same. `min_w_full` resolves against a block's outer width, padding and all (`forms::FormField`). In a flex row the host gives it `flex_1`; as a window's root it sits in a `size_full` box. A root centered under a cap keeps `w_full`, since `mx_auto` needs a width to fill (`layout::Container`); so does one that keeps a ratio, since taffy 0.9 follows a ratio from a set width, not a stretch (`layout::AspectRatio`, and `primitives::framed` under VideoPlayer, CameraPreview, ModelViewer, ParticipantTile and InpaintCanvas). A row that hands the text field element itself a `flex_1` share keeps `w_full` too: stretched without a width, taffy 0.9 can keep a pass that laid the share at no width, and the field draws nothing (`forms::Input`, `InputGroup`, `TagInput`); a row around one of those, as `NumberInput`'s is, takes none. `Button::full_width` is its host asking, since a button takes no style. Each chapter's test holds its filling roots at the full width of such a column through `layout::tests::narrow_width`, which reads a root by its `debug_selector`.
 - A field's `Highlight` sets color, wash, weight, slant and strike per span; measuring and painting share one set of runs, so wrapped heights match.
 - A block editor keeps one undo for the document: text and structure together, as snapshots through `forms::History`; it takes Undo and Redo before its fields do.
 - gpui calls focus listeners only while the window is active. A test that needs focus events activates its window with `window.activate_window()`.