T24d round 2 fix

- BrowserPreview keeps a ScrollHandle for its strip and a tracked focus
  handle per thumbnail; the focused thumbnail scrolls into view once, through
  forms::options::reveal and the canvas now shared as revealer
- Popup draws its reveal through the same revealer
- test tab_brings_a_hidden_frame_into_view

TASKS.md

@@ -280,3 +280,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T24c | 1 | FAIL | at 280px the path vanished and Accept all drew outside the box; the diff's fold rows took presses but no Tab |
 | T24c | 2 | PASS | |
 | T24d | 1 | FAIL | at 280px eight thumbnails ran past the browser card; the artifact panel lost its title and its controls ran out of the box |
+| T24d | 2 | FAIL | Tab to a later thumbnail left the strip where it was, the focused frame and its ring clipped |