Empty-Block Line-Break IME Browser Proof Plan

Goal

Replace the failed jsdom composition attempt with a real browser-capable IME proof for the empty-block line-break placeholder path.

What We Learned

• jsdom contenteditable composition is not a trustworthy proof seam here
• Slate already has Playwright integration under:
  • /Users/zbeyens/git/slate-v2/playwright/integration/examples
• Lexical already has the exact kind of IME coverage we need:
  • real browser tests in /Users/zbeyens/git/lexical/packages/lexical-playground/__tests__/e2e/Composition.spec.mjs
  • Chromium CDP Input.imeSetComposition usage for controlled IME composition

Chosen Next Cut

Build a Slate Playwright test for the placeholder example that:

1. starts from an empty editor with the placeholder visible
2. focuses the editor
3. drives composition with Chromium CDP Input.imeSetComposition
4. asserts the committed text result and final selection

That is the first honest way to decide whether the empty-block line-break path can drop FEFF in legacy code.

Why This Beats jsdom

• real contenteditable
• real browser selection
• real composition event ordering
• same class of harness Lexical uses for IME regression coverage

First Test Target

Use the existing Slate placeholder example:

• /Users/zbeyens/git/slate-v2/playwright/integration/examples/placeholder.test.ts

Add a new IME-focused case or a sibling test file that:

• uses Chromium only at first
• composes one Hangul or Hiragana syllable while placeholder is visible
• verifies:
  • final DOM text
  • final editor content
  • final selection position

Open Constraint

Do not widen legacy renderer policy until this browser proof exists and passes.