fix: ensure slideout cleanup completes without transition events

[mvanhorn]

Description

Make Slideout transition completion resilient when reduced motion is disabled but an expected transition event never arrives: retain the event-driven path, add a duration-aware fallback that invokes each completion callback exactly once, and treat targets with no effective transition as immediately complete. Track fallback work with the existing transition listeners so _cancelTransitionListeners() cancels both parts when a slideout is reopened or rapidly toggled; this prevents an obsolete close callback from hiding the active shade or container. Keep this logic inside the existing production-used _afterTransition() lifecycle rather than introducing a test-only seam, and preserve the current shade fade and container motion when events fire normally.

Craft.Slideout.close() hides its shade and outer container only from callbacks registered by _afterTransition(). When a browser does not emit the expected transitionend, those callbacks remain pending, leaving an invisible, full-viewport shade with display: block and active pointer events, and preventing the slideout's close lifecycle from finishing. The thread now includes a deterministic Craft.Slideout reproduction on Craft 5.10.13.2 and identifies the missing transition event, although the maintainer could not reproduce it in the same browser version and requested a safe-mode check. There are no assignees, competing PRs, or closed prior attempts in the supplied issue evidence.

Fixes #9912

Related issues

Not applicable to this change.

[Copilot]

Pull request overview

Adds reliable fallback cleanup when slideout transition events are missing.

Changes:

• Tracks and cancels transition fallback timers.
• Completes transitions once via event, timeout, or immediate reduced-motion handling.
• Adds Playwright coverage for cleanup and rapid reopening.

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated no comments.

File	Description
src/web/assets/cp/src/js/Slideout.js	Implements resilient, cancellable transition completion.
tests-playwright/tests/slideout/index.test.js	Tests completion, fallback, reopening, and reduced motion.

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[brandonkelly]

Thanks!

[brandonkelly]

Craft 5.11.0 is out with this change. Thanks again!