Add HeaderMenuOverlay, a plain in-window Rectangle rendered as the last
child of the AppShell root so it draws on top of the header and content
region. Unlike a PopupWindow it does not grab the pointer, so the header
tabs stay hoverable while a menu is open — hovering from one tab to
another overwrites HeaderMenuState and switches the menu instantly.
Behaviour preserved from the old PopupWindow menus:
- hover-to-open and click-to-open
- 1.5s idle auto-close, restarted on pointer activity over the panel
or its rows and on every tab (re)publish via a changed handler
- click-outside-close via a full-window transparent scrim
- Discover home-item row still switches to ContentView.home
- glyphs on all menus except Discover
Menu panel visuals are unchanged: surface-main bg, 1px border-muted,
Radius.sm, 24px drop shadow, 188px wide.
cc
vicrodh
5972e8052b