R1:42f1a95a1255

Fix a flash of document selection when "oncopy" and "inline on range" behaviors interact

Summary:
Ref T13513. In Safari, do this:

- view a 2-up diff with content on both sides;
- select more than one line on the right side; and
- use your mouse to click "New Inline Comment" in the context menu that pops up.

The mousedown event for the "New Inline Comment" click removes the "copy selection" behavior and creates a flash where both sides of the diff are selected.

This doesn't happen with (most)…