Fix onExit calling when the mouse is removed.

[gspencergoog]

Description

This PR solves two problems: currently, the onExit is called for a mouse pointer the moment the removal message is received, except that by the time it actually calls it, there is no _lastEvent for it in the mouse tracker (it's already been removed), resulting in an event being passed to the onExit that contains nulls for the position. Also, removePointer events don't actually get created with a position, although they easily could be, so that even the the _lastEvent in the mouse tracker were still populated, it would still give a null position and delta.

This PR adds support for the position and delta in a PointerRemovedEvent, and populates them. In addition, when a remove event is received, it doesn't actually remove the pointer until the mouse position check that gets scheduled actually happens.

Tests

I added the following tests:

• A Listener test that will check to make sure that an onExit gets the proper position.
• Modified the 'detects exit when mouse goes away' test in mouse_tracking_test.dart to verify that a position and delta is included.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
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Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[goderbauer]

If we already reported the delta on the hover event above, shouldn’t it be zero for the remove event then? My understanding is that it’s the delta of position since last event? (We should probably have a failing test for this situation...)

[gspencergoog]

Actually, I'm just going to remove the delta. It's not clear what it's a delta from, since there isn't another event of the same type to diff it from, and nobody's expecting a delta anyhow. And not specifying it will set it to zero in any case.

[goderbauer]

LGTM

[goderbauer]

Looks like Cirrus is showing a legit failure though?

[gspencergoog]

Yeah, fixed it.