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Slate React Chrome composition fallback must cancel model-owned overlap

docs/solutions/ui-bugs/2026-05-07-slate-react-chrome-composition-fallback-must-cancel-model-owned-overlap.md

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Original Source

すし` after a model-owned replacement should have won.

  • Chrome compositionend fallback appended stale committed composition text after an overlapping model change.
  • A too-tight ownership guard skipped legitimate DOM-current composition commits and dropped decorated text. root_cause: logic_error resolution_type: code_fix severity: high tags: [slate-v2, slate-react, ime, composition, model-owned, chrome]

Slate React Chrome composition fallback must cancel model-owned overlap

Problem

Chrome's compositionend fallback wrote committed IME text even after a model-owned command had already replaced the active composition range. The user would see stale composition text appended after the model command's result.

Symptoms

  • The new overlap proof expected first block text ---.
  • The first run produced ---すし.
  • The fallback still inserted event.data from compositionend after the model-owned replacement had superseded the composition.
  • A first attempt that required selectionSource === 'composition-owned' broke decorated spanning composition by producing albeta instead of alすしbeta.

What Didn't Work

  • Treating every compositionend payload as commit-worthy. That duplicates composition text after model-owned overlap.
  • Treating every non-composition-owned selection source as canceled. Real DOM composition can temporarily report dom-current, and still must commit.

Solution

Thread the input controller into composition-end handling and let the Chrome fallback skip model insertion only when the active composition was superseded by a model-owned command.

ts
const shouldCommitChromeFallback =
  ReactEditor.isComposing(editor) &&
  inputController.state.selectionSource !== 'model-owned'

commitChromeCompositionEndFallback({
  editor,
  rootElement: event.currentTarget,
  shouldCommit: shouldCommitChromeFallback,
  text: event.data,
})

When shouldCommit is false, still remove unmanaged composition DOM text. That keeps stale browser-owned text out of the rendered editor without writing it back into the Slate model.

Why This Works

During normal DOM composition, selection ownership can shift as the browser moves the DOM caret. That alone is not cancellation. A model-owned command is a stronger signal: Slate has deliberately moved or replaced the model selection, so Chrome's later fallback payload is stale for that range.

The narrow guard keeps legitimate DOM-current composition commits alive while dropping the stale payload after model-owned overlap.

Prevention

  • Pair overlap-cancel IME rows with adjacent normal composition rows; otherwise the fix can silently break DOM-current composition.
  • Assert model text, visible text, model selection, DOM caret, and composition trace for Chrome fallback changes.
  • Keep exact mobile-device claims separate. This proof is Chromium desktop browser behavior, not Android or iOS keyboard closure.