Panel dragger workaround for Safari

[TomTirapani]

Safari is not setting buttons or button to 1 on Drag events, which is causing our Panel resizing to early out.
See: #3797

This workaround adds a fallback check to the deprecated MouseEvent.which property, which is being set:
https://developer.mozilla.org/en-US/docs/Web/API/UIEvent/which#value_for_mouseevent_non-standard

As a deprecated API we obviously should not rely on it, but I think its safe to use as fall back for this specific issue in lieu of a better solution.

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[lbwexler]

Thanks for the quick turnaround here Tom. I think this is very pragmatic.