Slate browser Playwright helpers must normalize zero-width selection and wait for selection sync

What happened

The first real slate-browser Playwright tranche exposed two fake assumptions at once:

• the helper that claimed to return a Slate selection was just echoing native DOM offsets
• the helper that tried to prove clipboard copy contracts fired too early after Cmd/Ctrl+A

On the empty placeholder path, the browser caret really sits inside the FEFF marker at native offset 1.

That is fine for DOM assertions. It is wrong for Slate assertions.

On the rich copy path, DOM selection looked expanded immediately, but Slate’s copy handler still needed a short settle before synthetic copy captured the real payload.

Why this mattered

Without normalization, assertSelection(...) lies about Slate semantics on the exact zero-width path the browser framework is supposed to pin.

Without the settle, synthetic copy tests look random:

• DOM selection appears ready
• the copy payload is still empty
• the test blames clipboard transport when the real problem is selection sync

That is how bad helper APIs create bullshit failures.

What fixed it

1. Keep two different helpers with two different truths:
   • assertDomSelection(...) returns raw browser offsets
   • assertSelection(...) maps FEFF zero-width native offset 1 back to Slate offset 0
2. Treat selectAllInEditor(...) as asynchronous editor work, not an instant fact:
   • wait until selection is semantically ready
   • then give Slate a short settle before firing selection-sensitive synthetic copy assertions
3. Keep the clipboard contract test on DOM transport:
   • select real content in the editor
   • dispatch a synthetic copy event with an instrumented clipboardData sink
   • assert the HTML/plaintext payload written by Slate

Reusable rule

For Slate browser helpers:

• DOM selection helpers should expose raw native offsets
• Slate selection helpers must normalize zero-width marker offsets
• synthetic copy tests must wait for editor selection sync after Cmd/Ctrl+A

If a helper returns native FEFF offset 1 and calls it a Slate selection, or fires copy immediately after select-all, the helper is the bug.