bug(cdk/a11y): [Safari] [Chrome] [MacOS] FocusMonitor improperly identifies subsequent right mouse clicks as keyboard focus origin

This was found while stress-testing https://github.com/angular/components/pull/22489.

Reproduction

1. Go to https://material.angular.io/cdk/a11y/overview#focusmonitor on Chrome
2. Scroll to the “Monitoring focus with FocusMonitor” example.
3. Right click on “Child Button 2”. Mouse focus should be detected.
4. Now right click on “Child Button 1”. Observe that keyboard focus is incorrectly detected.

Expected Behavior

I expect mouse focus to be detected.

Actual Behavior

Keyboard focus was detected.

Cause

The second right-click event, on Chrome, is emitted with event.buttons === 0. This causes the isFakeMousedownFromScreenReader function to determine that it’s a fake mousedown, which causes FocusMonitor to attribute it as keyboard instead of mouse.

One idea: Can we update the isFakeMousedownFromScreenReader method to instead look at the event.offsetX and event.offsetY of the mouse event? That might be a better heuristic. I don’t have the context/history into this method though. @crisbeto @jelbourn

Environment

• Angular: Latest
• CDK/Material: Latest
• Browser(s): Chrome 89.0.4389.128
• Operating System (e.g. Windows, macOS, Ubuntu): macOS BigSur

[zelliott]

As for what the right behavior is, I think ideally we could match the behavior of :focus-visible, whatever that is for this case.

I did some quick testing:

• Subsequent right clicks disable :focus-visible (as expected, of course).
• Triggering a mouse click with VO (i.e. with VO + Shift + Space) disables :focus-visible.
• Triggering an element’s default action in VO’s virtual cursor (i.e. with VO + Space) is ignored by :focus-visible (i.e. it remains what it currently is).

Thus, my proposal seems to align with :focus-visible’s behavior.

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