fix(timeline): scroll stay-at-bottom, Jump to Latest button, and phantom unread regressions

[Just-Insane]

Description

A collection of scroll and UI regression fixes for the VList-based timeline:

1. Phantom unread dot + blank notification page recovery — fix stale unread indicators and empty notification view after room transitions.
2. useLayoutEffect for auto-scroll recovery after timeline reset — when sliding sync upgrades timeline_limit (1 → 50), a TimelineReset replaces VList content. The old useEffect fired after paint, causing a visible flash at the wrong scroll position. Switching to useLayoutEffect corrects scroll before the browser paints.
3. scrollToIndex for stay-at-bottom — use virtua's scrollToIndex instead of scrollToBottom for the stay-at-bottom behaviour, and remove a premature scroll call that raced with layout.
4. Timestamp-based settling window for Jump to Latest button — replace the boolean programmaticScrollToBottomRef guard with a timestamp + 200 ms settling window (SCROLL_SETTLE_MS). VList fires multiple intermediate onScroll events during re-measurement after scrollToIndex(); the old boolean guard cleared on the first isNowAtBottom=true callback, letting subsequent callbacks flash the button. The timestamp approach suppresses false negatives for the entire measurement pass.

Related to #635
Related to #55

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

AI assisted with diagnosing the scroll timing issues and writing the timestamp-based settling window. The settling window stores Date.now() when a programmatic scroll starts and ignores atBottom=false reports within 200 ms, letting VList's re-measurement pass complete without false negatives. AI also assisted with updating the useTimelineSync test to push a new event before TimelineReset so the useLayoutEffect auto-scroll recovery fires correctly.

[Copilot]

Pull request overview

Fixes several VList-based timeline regressions related to unread indicators, initial/auto scrolling, and “Jump to Latest” UI stability—especially around TimelineReset and sliding-sync timeline_limit upgrades.

Changes:

• Prevent “phantom unread” by excluding self-sent notification events from unread detection.
• Move auto-scroll recovery after timeline resets to useLayoutEffect and remove a premature pre-commit scroll.
• Replace bottom-pinning scroll logic with scrollToIndex(..., { align: 'end' }) and add a timestamp-based settling window to suppress transient “not at bottom” scroll callbacks.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/app/utils/room.ts	Adjusts unread detection to ignore self-sent notification events.
src/app/hooks/timeline/useTimelineSync.ts	Uses useLayoutEffect for post-reset scroll recovery; removes racing scroll call.
src/app/hooks/timeline/useTimelineSync.test.tsx	Updates TimelineReset anchoring test to better simulate real reset behavior.
src/app/features/room/RoomTimeline.tsx	Switches to scrollToIndex bottom pinning and adds timestamp-based settling guard + fallback “blank page” recovery effect.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The settling-window guard relies on programmaticScrollToBottomRef being updated before every programmatic scrollToIndex(..., { align: 'end' }) that is intended to keep the user pinned to the bottom. In this file there are still end-aligned scrollToIndex calls (e.g. in the top-spacer recalculation rAF and the post-pagination "wasAtBottom" branch) that do not set the timestamp, so the "Jump to Latest" button can still flash during Virtua re-measurement for those cases. Consider centralizing a helper (e.g. scrollToBottomWithSettle()) and using it for all bottom-pinning scrolls.

[Copilot]

This recovery effect can also run on a successful eventId load: liveTimelineLinked/eventsLength>0 can become true before the setIsReady(true) scheduled by the focusItem effects is committed, and then this effect will call scrollToBottom() and potentially override the intended scroll-to-event behavior. To avoid fighting the normal jump-to-event flow, gate this effect on an explicit failure signal (e.g. !timelineSync.focusItem) or a dedicated "load failed" flag from useTimelineSync.

Suggested change

	if (eventId && !isReady && timelineSync.liveTimelineLinked && timelineSync.eventsLength > 0) {
	scrollToBottom();
	setIsReady(true);
	}
	}, [
	eventId,
	isReady,
	if (
	eventId &&
	!isReady &&
	!timelineSync.focusItem &&
	timelineSync.liveTimelineLinked &&
	timelineSync.eventsLength > 0
	) {
	scrollToBottom();
	setIsReady(true);
	}
	}, [
	eventId,
	isReady,
	timelineSync.focusItem,