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Arc window may be cropped, which can cause showing/hiding
backdrop during the transition, which then may send
unnecessary synthesized mouse move even.

Bug: b/111319005
Test: Covered by unit test. Also tested on play music. See bug for repro step.

Change-Id: Ie030e24095eeadbd549e7cdc8cf7be0438c88d49
Reviewed-on: https://chromium-review.googlesource.com/1162705Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581086}