Description

This PR is an attempt to fix the Unexpected null value error thrown when using the Chrome DevTools on mobile mode and clicking on the top left corner.

Before this PR, the error is thrown because the pointerup handler expects that there was a previous pointerdown handler which called _ensureSanitizer for the device.

This is not the case when opening DevTools and switching it to mobile mode, because :

• Chrome DevTools sends event of PointerDeviceKind.touch instead of PointerDeviceKind.mouse and create a new pointerId on each pointer down event.
• Chrome DevTools set the event target to the html DOM element when the event is on the top left corner (elsewhere the target is Flutter Web glass pane). This is related to the <flt-semantics-placeholder> DOM element (the problem doesn't occur when this element is removed).

The fix included in this PR is to ignore pointer up and pointer cancel events when the device is unknown. This is to improve the developer experience (especially when the debugger is configured to break on Uncaught Exceptions).

Alternative solutions :

• Digging in Chrome DevTools code (seems too complex for this case).
• Changing how the Semantic placeholder is created (it doesn't seem sensible to change it for this edge case and it is not obvious which change can be made).

Related Issue

Fixes flutter/flutter#105458

Tests

Adds 1 test.