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Because Linux/CrOS do not respect hit test transparency, the top shadow
of bubbles fights for mouse events with the window underneath. This
causes unacceptable flicker with tab hover cards on Linux and ChromeOS.

As a temporary fix until the real culprit can be addressed (a much
larger effort; see crbug/978134), on the affected platforms we move
hover cards down slightly and slightly raise the bottom of the hit test
area for tabs up slightly to compensate. This is not ideal from a UX
standpoint but it is the only solution available at the current time for
an issue that is otherwise a release blocker.

Bug: 972582
Change-Id: I91f0583f8c6c4dfb912c4ea75e07a437fc770f02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674290
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671885}