[trissim]

The saved-history fixture registered saved but kept the selected tab unbound, attached a processless agent, and handed that unrelated tab a source page. The UI correctly refused to claim retained coverage.

The fixture now opens a MailboxTab for the registered owner's exact incarnation and root through session_navigation.new, with MainScreen.from_thread as the screen constructor. That constructor calls bind_thread (screens/main.py:301-310), which installs ThreadIdentity(owner, root) before the history widget mounts. The normal bound source loads and publishes its own saved history; the fixture waits for that coverage before reading the page used for later park/revalidation checks.

• Deleted the detached-agent attachment and initial bare received(page) injection.
• Kept every saved-history availability check and all assertions about parked coverage, retained pages and retained child widgets.
• Updated the two stale reveal_retained calls to the existing BoundView method with (owner, history).
• Replaced the removed window.histories lookup with identity equality against the bound view's history.
• Title checks still require the same exact text; str(details.title) reads Textual Content's actual text instead of applying string membership to Content's character sequence.

Based on main f9aa367, which merged #1169. Only tests/textual/test_saved_history_coverage.py changes. No private attribute mutation or patch of application code. git diff --check passes; no tests run.

Assumed: execution is checked by the review fork; the fixture's eight-row saved journal remains the retained source used by those checks.

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