It is a bit like a sobriety test to get the action menu to open and actually select something. Any diagonal mouse movement (as opposed to moving exactly horizontal) from the hover button over to the popup menu will dismiss the menu. Additionally, if you happen to move slightly left back toward the button, it will also dismiss the popup.

Generally with this sort of thing, you create a larger virtual active area as soon as the popup is open (which includes the button, the popup, and a little buffer around the whole thing) and you need to move outside of that to dismiss the popup. For bonus points, you provide a timer where you are allowed to leave the active area for a short duration before the popup dismisses, this handles the overshoot case.

I set the priority to 'minor' but really, creating pull requests easily is a pretty major function in the product.