fix(code): hydrate transcript tail when scrolled to the bottom edge

[Mason Daugherty (mdrxy)]

Fix newest transcript messages sometimes failing to reappear when scrolling to the bottom of a long, virtualized chat history.

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test_scroll_down_hydrates_tail_below was flaky (~30% TimeoutError) because scroll-driven tail hydration compared the message store's estimated bottom_spacer_top against the real DOM scroll geometry (scroll_y/max_scroll_y). When estimated spacer heights drift from the rendered layout — which depends on the viewport height Textual happens to allocate — the distance heuristic can sit exactly at its threshold while the view is already parked at max_scroll, so should_hydrate_below returns False forever and the archived tail is never remounted.

should_hydrate_below now hydrates whenever the view is at the scroll edge but history remains archived below, mirroring how scrolling to offset 0 always hydrates above. This is a real user-facing fix, not just a test tweak: a user who scrolls to the very bottom while the tail is archived would otherwise be stranded on a blank spacer.

Made by Open SWE

[open-swe]

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