[cp:stable][web] Work around wrong pointerId in coalesced events in iOS Safari 18.2

[mdebbar]

Issue Link

#158299

Target

stable

Cherry pick PR Link

flutter/engine#56903

Changelog Description

Manual cherrypick to workaround a pointer/gesture bug in iOS 18.2 Safari.

Impacted Users

Users on iOS 18.2

Impact Description

The most obvious impact is scrolling. In iOS 18.2, Flutter Web don't scroll. Other gestures may also be impacted, but scrolling is the most obvious one.

Workaround

This cherrypick is the workaround :) Unfortunately, there's nothing users can do to workaround this iOS bug.

Risk

low

Test Coverage

yes

Validation Steps

No response

[christopherfujino]

This was merged in Flutter 3.27.2 stable.

[github-actions]

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