[popups] Dismiss after dropped IME compositions

[jjenzz]

In #5967, Escape stops dismissing a popup after Chromium abandons an IME composition without emitting compositionend. This change restores dismissal after that abandoned composition while continuing to protect Escape used by an active IME.

An Input Method Editor (IME) lets someone enter text such as Japanese or Chinese using a Latin keyboard. A composition is the temporary editing stage before that text is committed—for example, typing a pronunciation and choosing the intended characters. During this stage, Escape can cancel the composition, so it should not also dismiss the popup. MDN's composition event explanation and interactive example shows how this works in the browser.

Closes #5967

Root cause and solution

The dismissal handler records compositionstart and assumes compositionend will clear that state. Removing the composing input or rewriting its contents can omit the end event, leaving the flag set indefinitely. Every later Escape is then treated as composition input.

Active composition is now identified from the keyboard event's isComposing or keyCode === 229 indicators. The start listener and its unbounded flag are removed. The existing self-expiring compositionend suppression window remains, preserving the Safari end-before-key safeguard.

The event describes the current Escape, so an abandoned composition cannot leave stale state behind. Clearing the old flag only when an input disconnects would miss aborts caused by rewriting a connected input. Relying exclusively on native flags would discard the existing Safari timing protection. The selected solution handles both without DOM observers, mutation interception, or consumer coordination.

Validation

• Native CDP traces in Chromium 153.0.8010.12 on macOS confirm that input removal, input value replacement, and contenteditable replacement omit compositionend. The following Escape reports isComposing=false/keyCode=27; active composition Escape reports isComposing=true.
• Native Japanese Hiragana testing in Firefox 157 on macOS reproduced the dropped-end bug: removing the composing input omitted compositionend, then Escape reported isComposing=false/keyCode=27. The original implementation kept the dialog open; the fix closed it. Active composition Escape preserved the dialog.
• Native Japanese Hiragana testing in Safari 26.6.1 on macOS confirmed that Escape preserves the dialog during active composition and dismisses after committing. Rewriting a connected input emitted compositionend, and subsequent ordinary Escape dismissed normally.
• Shared popup conformance verifies dismissal after a dropped end event and non-dismissal for each native IME guard. Original source fails the new regressions; removing either guard independently fails its corresponding tests.
• Existing composition timing and close/reopen tests pass. jsdom: 421 passed, 175 skipped. Chromium popup roots: 526 passed, 3 skipped.
• Full TypeScript, ESLint, stylelint, and Prettier checks passed.

Public APIs and controlled dismissal/cancellation behavior are unchanged. The keyCode === 229 guard follows the existing Base UI input/list handling.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5978
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5978
  

commit: fb9b814

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	▼-42B(-0.01%)	🔺+5B(0.00%)

Details of bundle changes

Performance

Total duration: 471.25 ms -66.58 ms^(-12.4%) | Renders: 76 ⁽⁺⁰⁾ | Paint: 865.29 ms -100.74 ms^(-10.4%)

Test	Duration	Renders
Mixed surface mount (app-like density)	40.60 ms ▼-12.83 ms(-24.0%)	5 (+0)
Scroll Area mount (300 instances)	40.95 ms ▼-12.12 ms(-22.8%)	3 (+0)
Select open (500 options)	27.36 ms ▼-7.60 ms(-21.7%)	14 (+0)
Combobox type — 500 items, all stay mounted (type "Row ")	12.75 ms ▼-4.52 ms(-26.2%)	11 (+0)

11 tests within noise — details

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