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Slate v2 placeholder IME proofs must commit on compositionend

docs/solutions/logic-errors/2026-04-04-slate-v2-placeholder-ime-proofs-must-commit-on-compositionend.md

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Slate v2 placeholder IME proofs must commit on compositionend

What happened

The first real-browser slate-v2 placeholder IME proof started red for the right reason:

  • the empty placeholder path rendered a FEFF zero-width span plus
  • Chromium IME composition reached the surface
  • the final Slate selection and text were wrong

At first the example reconciled the editor from native input on every composition step.

That looked reasonable and was wrong.

It turned transient composition text into committed document state, so the DOM ended up as the concatenation of every IME step instead of the committed text.

What didn't work

  • Normalizing the bridge harder did not fix it. The bridge was not the main liar anymore.
  • Replacing editor state on every insertCompositionText input broke the live composition region and appended each next IME step.
  • Waiting for a final friendly insertText input was also bullshit on the clean path. Once transient composition input stopped forcing rerenders, Chromium finished with compositionend and no later commit-friendly input event.

What fixed it

Treat transient composition and committed text as different phases:

  • ignore input events while event.isComposing is true
  • ignore insertCompositionText / deleteCompositionText
  • reconcile the editor from DOM on compositionend
  • still reconcile from normal input for non-IME text entry

That made the proof surface behave honestly:

  • the DOM kept the live composition text inside the zero-width placeholder path
  • the editor only committed once composition was finished
  • the final browser proof landed at text すし with selection 0.0:2|0.0:2

Why this works

IME composition text is not stable document truth.

During composition, the browser owns a temporary native region and may rewrite it several times before commit. If the proof surface copies that transient DOM back into Slate on each step, it destroys the composition lifecycle it is trying to measure.

compositionend is the first trustworthy point on this Chromium path where the final committed text is already in the DOM and can be translated back into the editor model without duplicating intermediate steps.

Reusable rule

For slate-v2 browser proof surfaces on IME-sensitive placeholder paths:

  • do not commit transient composition text into the editor model
  • do not assume a final commit-friendly input event will arrive after IME
  • treat compositionend as the commit boundary unless the concrete browser proof shows a different contract

If a proof surface reconciles on every composition input, it is testing its own bad input policy instead of the editor seam.