fix(threads): skip classic-sync backfill when server-side thread support is active

[Just-Insane]

Description

Fixes a flood of EventTimelineSet.addEventToTimeline: Ignoring event=… that does not belong in timeline=… timelineSet(threadId=…) console warnings that appeared whenever a thread was opened in rooms with server-side thread support enabled (the default on Synapse/element.io homeservers).

Root cause: PR #343 added a backfill useEffect in ThreadDrawer that calls thread.addEvents(liveEvents, false) when the Thread object had no replies (classic-sync workaround). However, when server-side thread support is active thread.initialEventsFetched starts as false — meaning the SDK is already going to call updateThreadMetadata() → timelineSet.resetLiveTimeline() + client.paginateEventTimeline() automatically. Calling thread.addEvents() ourselves before that completes causes events to be inserted into a timeline that is mid-reset, so canContain() fails for every event → warning flood.

Fix: Guard the backfill with if (!thread.initialEventsFetched) return. This is false exactly when Thread.hasServerSideSupport is set, so the backfill now only runs in the pure classic-sync case it was designed for.

Fixes #

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

AI assistance level

• None — No AI tools were used
• Research only — AI used for research, brainstorming, or understanding existing code; no generated code included
• AI-assisted — AI suggested code or logic that I reviewed, tested, and understand fully; I can explain every change
• Primarily AI-generated — The majority of code was AI-generated; I have thoroughly reviewed it, verified correctness, and made it my own

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

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