The lifecycleKeepAlive gate in watchPreviewTileMirror had no test: swapping
preview-tile.tsx back to main left the suite green, so a regression that
dropped the flag (and turned Hide back into an unmounting Minimize for the
in-app browser) would have shipped silently. preview-tile.test.ts now drives
the real mirror and asserts a url tile registers with lifecycleKeepAlive=true
while a text file peek stays evictable.
The Hide label and kept-mounted hidden body key on lifecycleKeepAlive for every
pane, and the Terminal pane sets it too; the user guide only mentioned the
browser, so the sentence now covers both.
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