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Slate browser IME proof rows need honest DOM composition boundaries

docs/solutions/developer-experience/2026-05-07-slate-browser-ime-proof-rows-need-honest-dom-composition.md

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Original Source

Slate browser IME proof rows need honest DOM composition boundaries

Problem

The Mobile/IME proof lane needed a row for composition inside marked text. Using Chromium CDP IME as the oracle for a mid-leaf mark boundary looked close, but it was not honest for this harness.

Symptoms

  • Model selection and DOM selection both let Input.imeSetComposition commit at the end of bold rich, producing richすし instead of riすしch.
  • A replacementStart / replacementEnd experiment inserted after Th, proving the offsets were not the Slate leaf offsets the test needed.
  • Editing packages/slate-browser/src/playwright/ime.ts did not change Playwright behavior until packages/slate-browser was rebuilt, because tests import slate-browser/playwright from dist.

What Didn't Work

  • Treating editor.selection.select(...) as enough for native CDP IME.
  • Treating editor.selection.selectDOM(...) as enough for native CDP IME.
  • Passing local leaf offsets to CDP replacement fields.
  • Patching slate-browser source and rerunning Playwright without rebuilding the package.

Solution

For the mark-boundary row, use the same proof shape as ProseMirror's webtest-composition.ts: set a real DOM range in the marked text node, dispatch composition events, mutate the captured range, then let Slate import the DOM change.

The landed row asserts the actual Slate contract:

  • composing inside bold rich at offset 2 produces riすしch
  • the bold mark remains intact
  • selection advances inside the marked leaf
  • composition trace ownership is recorded
  • Backspace recovery restores the original marked text and caret

When changing reusable slate-browser helpers, rebuild before expecting Playwright to see the edit:

bash
cd /Users/zbeyens/git/slate-v2/packages/slate-browser
bun run build

Why This Works

The proof row is about Slate importing a browser-owned composition mutation without corrupting marks or selection. A direct DOM composition mutation proves that contract. CDP IME is still useful for end-of-text native composition and trace rows, but this harness did not provide reliable mid-mark replacement semantics through CDP offsets.

Prevention

  • Do not promote CDP IME rows to exact mid-mark or mobile-device claims unless the insertion point itself is proved.
  • For ProseMirror composition rows, translate the DOM-mutation contract first; add native CDP coverage only when its offset semantics are proved for that surface.
  • If a Playwright test imports slate-browser/playwright, remember it is using built package output.