Handle out-of-order add/remove pointer events

[hbatagelo]

Fixes #146251.

On Windows, a "pointer add" event to a new view is sometimes sent before the "pointer remove" event from the old view. When PointerDataPacketConverter handles the add event, it asserts that the device's pointer state has been cleared. Since the pointer state is cleared only when the remove event is processed, the assertion fails if the events arrive out of order.

The solution proposed here is to synthesize a remove event if an add event is received while the pointer state hasn't been cleared (i.e., if the pointer is added without being removed from the previous view).

To avoid duplicate remove events, the view ID is now tracked in PointerState. If the original "remove" arrives after the synthesized one, the state's view ID will differ from the pointer data's view ID, allowing it to be safely ignored.

When synthesizing the remove event, it's possible that the old view has already been destroyed, meaning no remove event will be received later. For example, this occurs when a window is destroyed while the pointer is inside it. However, the framework expects an "add" event to always follow a "remove" event, and "remove" events with invalid views are ignored. To meet the framework's expectations, the view ID of the "add" event will be used for the "remove" event if the old view has been destroyed.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[chinmaygarde]

Triage: @loic-sharma, do you how to direct this to reviewers?

[loic-sharma]

I'm not very familiar with this code. Could you explain why the cancel event is synthesized only if the view exists? That's not immediately obvious to me

[hbatagelo]

I replicated the behavior of the non-synthesized/original remove event. When a non-synthesized "remove" is handled, a "cancel" is synthesized if the pointer is down, and a "hover" is synthesized if the pointer location has changed. Here, I replicate the synthesized "cancel" event, but not the "hover" event, since the pointer location is unchanged.

My reasoning for synthesizing the "cancel" event only for valid views is that the framework will ignore events for non-existing views.

[loic-sharma]

Thanks for the excellent contribution! This patch looks good to me, but I'm not very familiar with this area. @dkwingsmt or @moffatman could you give this a review?

[loic-sharma]

FYI, your PR failed the following checks:

Errors...

[5735/6381] ACTION //flutter/lib/snapshot:generate_snapshot_bin(//build/toolchain/mac:ios_clang_arm)
FAILED: gen/flutter/lib/snapshot/vm_isolate_snapshot.bin gen/flutter/lib/snapshot/vm_snapshot_instructions.bin gen/flutter/lib/snapshot/isolate_snapshot.bin gen/flutter/lib/snapshot/isolate_snapshot_instructions.bin 
vpython3 ../../build/gn_run_binary.py clang_arm64/gen_snapshot --snapshot_kind=core --enable_mirrors=false --vm_snapshot_data=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/vm_isolate_snapshot.bin --vm_snapshot_instructions=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/vm_snapshot_instructions.bin --isolate_snapshot_data=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/isolate_snapshot.bin --isolate_snapshot_instructions=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/isolate_snapshot_instructions.bin /Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/flutter_patched_sdk/platform_strong.dill
Command failed: ./clang_arm64/gen_snapshot --snapshot_kind=core --enable_mirrors=false --vm_snapshot_data=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/vm_isolate_snapshot.bin --vm_snapshot_instructions=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/vm_snapshot_instructions.bin --isolate_snapshot_data=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/isolate_snapshot.bin --isolate_snapshot_instructions=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/lib/snapshot/isolate_snapshot_instructions.bin /Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/flutter_patched_sdk/platform_strong.dill
exitCode: -9

[5736/6381] ACTION //flutter/testing/scenario_app:_scenario_app_snapshot_snapshot(//build/toolchain/mac:ios_clang_arm)
FAILED: gen/flutter/testing/scenario_app/ios/snapshot_assembly.S 
vpython3 ../../build/gn_run_binary.py clang_arm64/gen_snapshot --deterministic --snapshot_kind=app-aot-assembly --assembly=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/testing/scenario_app/ios/snapshot_assembly.S /Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/testing/scenario_app/kernel_blob.bin
Command failed: ./clang_arm64/gen_snapshot --deterministic --snapshot_kind=app-aot-assembly --assembly=/Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/testing/scenario_app/ios/snapshot_assembly.S /Volumes/Work/s/w/ir/cache/builder/src/out/ios_profile/gen/flutter/testing/scenario_app/kernel_blob.bin
exitCode: -9

[19/104] ShellTest.EncodeImageRetryOverflows (986 ms)
[INFO:flutter/testing/test_timeout_listener.cc(76)] Test timeout of 300 seconds per test case will be enforced.
�[0;33mNote: Google Test filter = ShellTest.EncodeImageRetryOverflows
�[m�[0;32m[==========] �[mRunning 1 test from 1 test suite.
�[0;32m[----------] �[mGlobal test environment set-up.
�[0;32m[----------] �[m1 test from ShellTest
�[0;32m[ RUN      ] �[mShellTest.EncodeImageRetryOverflows
[19/104] ShellTest.EncodeImageRetryOverflows returned/aborted with exit code -11 (986 ms)

These failures don't look related to your change. I've retried these failing checks. If the re-runs fail, I would try rebasing your changes off the latest main commit.

[dkwingsmt]

Awesome change with awesome doc, thank you so much!

[loic-sharma]

Thanks for the wonderful contribution!