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Slate React iOS Korean Backspace must stay native-owned

docs/solutions/ui-bugs/2026-05-07-slate-react-ios-korean-backspace-must-stay-native-owned.md

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Original Source

Slate React iOS Korean Backspace must stay native-owned

Problem

Lexical explicitly lets native iOS Korean Backspace behavior run instead of turning the keydown into an editor-owned delete command. Slate's keyboard runtime did not have the same guard, so a Korean iOS Backspace could be claimed by the model-owned keydown path before native input behavior had a chance to apply.

Symptoms

  • Local Lexical source has matching guards in plain text and rich text command registration for IS_IOS && navigator.language === 'ko-KR'.
  • Slate's keyboard-input-strategy.ts resolved Backspace to a destructive command and called preventDefault() before any platform/language exception.
  • The fix needed to stay honest: this is a runtime contract proof, not exact raw-device Korean keyboard closure.

What Didn't Work

  • Treating the existing Android beforeinput unit rows as coverage. Android model-owned Backspace and iOS Korean native Backspace are opposite ownership decisions.
  • Adding a broad mobile exception. The Lexical evidence is narrow: iOS plus ko-KR plus backward delete.
  • Claiming a raw-device fix from a package unit test. The row protects the runtime decision; device closure still needs matching iOS evidence.

Solution

Add a narrow keyboard runtime predicate and call it before destructive Backspace becomes model-owned:

ts
export const shouldDeferBackspaceToNativeInput = ({
  isIOS = IS_IOS,
  language = typeof navigator === "undefined" ? "" : navigator.language,
  nativeEvent,
}: {
  isIOS?: boolean;
  language?: string;
  nativeEvent: KeyboardEvent;
}) => isIOS && language === "ko-KR" && Hotkeys.isDeleteBackward(nativeEvent);

Then return keyDownUnhandled() for that case, letting the native browser input path own the action.

The regression row lives in .tmp/slate-v2/packages/slate-react/test/keyboard-input-strategy-contract.test.ts and asserts:

  • iOS + ko-KR + Backspace defers to native input;
  • iOS + non-Korean Backspace remains model-owned;
  • iOS Korean non-Backspace keys remain model-owned.

Why This Works

The bug is an ownership error, not a delete algorithm error. Korean iOS input is one of the cases where native behavior carries IME-specific state that the editor's generic Backspace command should not preempt. Keeping the predicate narrow preserves normal model-owned Backspace behavior everywhere else.

Prevention

  • When stealing mobile/IME behavior from Lexical or ProseMirror, classify whether the reference system is choosing native ownership or editor ownership before writing the Slate row.
  • Keep platform/language exceptions as predicates with focused contract tests, not inline comments buried in the event handler.
  • Do not promote package-level platform predicates to exact iOS keyboard claims without raw-device or matching simulator/manual proof.