fix Firefox arrow navigation bug

[jmusial]

Fixes navigation bug on Firefox, where navigation using arrows over emojis requires 2 key presses.

Screen.Recording.2025-05-21.at.09.55.11.mov

Details

In Firefox, the arrow key navigation treats surrogate pairs (like emojis) as two separate code units, while Chromium browsers treat them as a single character. This discrepancy causes the “double-arrow press” behaviour.

Related Issues

GH_LINK

Manual Tests

1. Open editor with some emojis typed in
2. Try navigating with your arrows
3. Results should be consistent across all browsers and 1 key press should move cursor over 1 emoji

Videos

Before

LM_emojis_before.mov

After

LM_emojis_after.mov

Linked PRs

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[jmusial]

recheck

[jmusial]

While testing noticed that the issue is also present for left arrow when navigating backwards and /or with multiple emojis. Added code and test cases to cover that.