ui: keep a hidden sidebar from intercepting clicks again (#32035)
The hidden desktop strip relied on inherited `visibility: hidden` to keep
its content from catching clicks. Since the sidebar's sections fold
(#31936), an open section's CollapsibleRegion sets `visibility: visible`
again, and its rows' context-menu triggers carry an inline
`pointer-events: auto`, so they caught clicks meant for the page.
Make the hidden strip's content inert, which no descendant can undo, and
add a client test that hit-tests the hidden sidebar's area.
Fixes #31971
Assisted-by: Claude (Opus 5.5 Medium)