Fix IME composition Enter key handling on Windows

[PttCodingMan]

Summary

Fixes a bug where pressing Enter to confirm an IME selection on Windows would trigger a spurious block split in the editor, causing the "line shifts down" symptom. The issue occurs because Windows fires a stray keydown event for Enter right after compositionend, which ProseMirror's default handler interprets as a deliberate newline.

Changes

• New module imeGuard.js: Implements isStrayPostCompositionEnter() to detect and suppress only the stray Enter that arrives within 50ms of compositionend. The logic carefully preserves Enters that are still part of an active composition (e.g., Zhuyin pressing Enter to commit phonetic input) and those delivered to the IME itself (keyCode 229).

• New test file imeGuard.test.js: Comprehensive test suite covering:

  • Suppression of stray Enter within the 50ms window
  • Preservation of Enters during active composition
  • Preservation of Enters delivered to the IME
  • Rejection of Enters outside the window
  • Edge case: regression test for the NO_COMPOSITION sentinel to prevent false positives during page load

• Updated Editor.jsx: Replaced the previous composition guard logic with a cleaner implementation:

  • Removed the compositionstart handler (no longer needed)
  • Changed compositionend to record a timestamp (lastCompositionEndAt) instead of setting a boolean flag
  • Replaced the boolean check with a call to isStrayPostCompositionEnter() that uses the timestamp window
  • Removed the setTimeout(0) workaround in favor of a time-based window

Implementation Details

• Uses performance.now() for precise timing, consistent with the timestamp recorded at compositionend
• The 50ms window is short enough to catch the stray Enter (typically arrives within a few ms) but long enough to avoid false positives from deliberate newlines
• The NO_COMPOSITION sentinel is Number.NEGATIVE_INFINITY to prevent false positives during the first 50ms after page load (when performance.now() is small)
• Respects IME-specific requirements: never suppresses Enters with isComposing: true or keyCode: 229, as these are part of the IME's input flow

https://claude.ai/code/session_01KT7qFP6oQ5GdiotRi7X5De