fix(mobile): stop thread down-arrow flicker

[CharryLee0426]

Fixes #7314. Related: #6807.

Stops the down-arrow button from flashing when opening mobile threads with long messages. The thread settles before it becomes visible, and the arrow stays stable during automatic scrolling. It still appears when there is content below the screen and lets you jump to the bottom.

Testing

• Manually tested on iOS and Android.
• just mobile-check passed.
• just mobile-test passed: 2,105 tests, including regressions for thread entry, scrolling, new replies, deep links, and the jump-to-bottom button.
• Full just ci stopped at the unchanged desktop test useKnownAgentPubkeys.test.mjs; that test passed when rerun on its own.

Before the fix

01_before_installing_fixed_ver.mp4

Fixed version: end-to-end test

02_fixed_version_e2e.mp4

Reinstalled published version: comparison

03_reinstalled_online_ver.mp4

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 813bbd14121edacc6cb4733301a3af12131aa10e...282ca690a50afe4d8bfbff75d24115ce77fbc085.
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[CharryLee0426]

@klopez4212 could you take a look when you have a chance? This fixes #7314 and relates to your jump-to-latest work in #6807. I’ve included before/after recordings, tested on iOS and Android, and added regression coverage for thread settling and arrow visibility.