The desktop app and an installed PWA had a row across the whole window
with the title and the window controls. That row is gone. The rail,
sidebar, list and reader run to the top of the window in their own
colours, and the controls sit in the top row of whatever is rightmost.
- `lib/window-controls-overlay.ts` finds and marks those rows before
paint, and they reserve the controls' width through
`--titlebar-right-inset`
- the rows and the rail are the drag area, everything clickable is
`no-drag`, and an invisible band lets a page with no row still move
- settings and the admin shell get a row only under the overlay, Pro's
tab strip and Mountain View's bar match the controls' height
- Alt floats the menu over the top left, in `window-menu-bar.tsx`
- skeletons carry the same marks, so nothing shifts when the app loads
A frame slot `side-panel`: a plugin's panel right of the mail list and
reader, opened from a button in the panel rail (Mountain View's, or a slim
rail in the default layout while a plugin offers one). `ui.setSidePanel`
names its button; `ui.openPanel('side-panel')` opens it.
`ui.setSidePanel` takes `rail: false`: the rail draws no button for the
plugin's panel, which then opens only on the plugin's request
(openPanel / togglePanel('side-panel')), e.g. from a top bar action. A
rail left without buttons is not drawn, in the default layout and in
Mountain View's side panel alike.
Linus Rath
Jacob Zeitz