[flutter-zl]

Fixes #194104

Problem

On mobile, the engine covers each view with a semantics placeholder and removes it after 20 events that do not activate a screen reader. On non-WebKit browsers it removed the placeholder on the very next event, usually a pointermove in the middle of a drag. That placeholder is the drag's target, and when the page lets the browser pan and cancels touchmove, Chrome on Android stops delivering the rest of the drag, including its pointerup, so the framework keeps the pointer down forever.

Fix

Remove the placeholder only on touchend, pointerup, or click, once the gesture that targets it has ended, on every browser engine. WebKit has used this rule since the placeholder was added. Accessibility was already given up on, so the only change is that the placeholder goes away when the finger lifts instead of mid-gesture.

Demo

On an Android phone in Chrome, reload the page, do one long, slow drag on the list, and lift the finger.
Before, "Pointers down" stays at 1: https://flutter-demo-77-before.web.app
After, "Pointers down" returns to 0 and the list is idle: https://flutter-demo-77-after.web.app

[gemini-code-assist]

Code Review

This pull request modifies the semantics placeholder removal logic in MobileSemanticsEnabler to trigger on touchend, pointerup, or click events across all browser engines, ensuring the targeting gesture has completed before removal, and adds a test for this behavior. The review feedback recommends extending this logic and documentation to also handle touchcancel and pointercancel events, preventing placeholders from remaining in the DOM if a gesture is canceled.

[gemini-code-assist]

Update the documentation to also mention touchcancel and pointercancel events, as they also signal the end of a gesture.

Suggested change

	/// The placeholders are removed upon the next "touchend", "pointerup", or
	/// "click" event, once the gesture that targets them has ended. Removing them
	/// The placeholders are removed upon the next "touchend", "pointerup",
	/// "touchcancel", "pointercancel", or "click" event, once the gesture that
	/// targets them has ended. Removing them

[flutter-zl]

Done.

[gemini-code-assist]

The gesture can also end due to cancellation (e.g., when a system gesture takes over or the touch is disrupted). In these cases, pointercancel or touchcancel events are fired instead of pointerup or touchend. If we do not remove the placeholder on cancellation, it will remain in the DOM and potentially block subsequent interactions. We should include pointercancel and touchcancel as events that trigger placeholder removal.

Suggested change

	event.type == 'touchend' || event.type == 'pointerup' || event.type == 'click';
	event.type == 'touchend' ||
	event.type == 'pointerup' ||
	event.type == 'click' ||
	event.type == 'touchcancel' ||
	event.type == 'pointercancel';

[flutter-zl]

Done.