Switching A -> B -> A unmounted A's Browser (its pane contribution was
disposed), so returning made a new Electron guest: unsaved input and page
JS state were lost, where main kept them. A preview tab whose session leaves
the screen now leaves the layout tree but keeps its contribution, so its
keep-alive host stays mounted (hidden, inert, aria-hidden, no pointer
events) and the same guest re-docks on return. Parked panes are excluded
from tree adoption (parked-panes.ts). Retention is capped at 8 hidden live
pages, longest-hidden first; past the cap a page unmounts and reloads from
its saved row when its session returns. Agent tools are unaffected: they
resolve tabs by owner, never by what is mounted.
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