Support the mouse in the branch popup

A click on a branch switches to it, the wheel moves the selection, a click
outside closes the popup. The popup is hit-tested by its own position, as it
is drawn after zone.Scan.

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

docs/ux-guide.md

@@ -353,6 +353,9 @@ where the palette can't be shown, see [Key bar and palette](#key-bar-and-palette
 - Click a row: select it. Click the header: select the superproject.
 - Click `▸` / `▾`: select and toggle.
 - Scroll wheel: move the selection.
+- In the [branch popup](#switch-branch): click a branch to switch to it,
+  wheel to move, click outside to close. The popup is hit-tested by its own
+  position, not with bubblezone, since it is drawn after `zone.Scan`.
 
 ## Accessibility and terminals