Firefox Playwright keyboard insertText can provide a stronger-than-proxy composition lane

What happened

The existing non-Chromium IME helper used a pure proxy path:

• dispatch synthetic compositionstart
• dispatch synthetic compositionupdate
• mutate the DOM directly
• dispatch synthetic compositionend

That was enough for a fallback lane, but it was still proxy-shaped.

On Firefox, page.keyboard.insertText('すし') turned out to be materially better on the debug IME surfaces.

Observed event trail on the placeholder row:

• compositionstart
• compositionupdate:すし
• beforeinput:insertCompositionText:すし
• compositionend:すし
• input:insertCompositionText:すし

The same direct pattern also worked on:

• no-FEFF placeholder
• inline-edge
• void-edge

with the expected committed Slate state.

Why this matters

This is stronger than the old proxy lane because the browser is generating the actual input events instead of the test mutating the DOM tree by hand.

It is still not “real user IME on real hardware,” but it is no longer just synthetic composition theater.

Reusable rule

For Firefox Playwright IME follow-up:

• try page.keyboard.insertText(...) before accepting that proxy is the ceiling
• if the debug surface shows beforeinput:insertCompositionText:* and input:insertCompositionText:*, count that as stronger-than-proxy evidence
• keep the old proxy helper as a backstop for WebKit or other browsers that do not expose the same behavior