Firefox doesn't consider mousedown to be a user gesture.  This seems to be the right model because a mousedown is not a reliable indicator that the user intends activate something.

One user behavior that particularly supports this argument is that when a user accidentally/inadvertently presses the mouse button on a link/button, it is common to avoid a click (activation) by dragging the mouse pointer out of the button/link and then releasing the mouse button.

In Chrome we did a similar fix for touchstart:  Issue 611981 .

A secondary motivation for wanting to remove mousedown user gesture is that this has resulted in a complicated token passing in our mouse event handling because a mousedown and mouseup must share a single token.  One particular complication: mousedown gets handled by EventHandler before the event goes to a plugin which acquires the mouse lock, as a result the mouseup goes to the plugin directly.  Now that plugin mouseup code should share Blink's mousedown token!

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If we agree that this is the right thing to do, we will also need to convince both Edge & Safari to do the same.