Mouse events are grabbed for too long on Mac when a control invokes a pop-up menu

TO REPRODUCE: (Mac OS X only)

1) Create an app with a few controls, one of which is a QComboBox with some items in it
2) Start the app
3) Click on the combo box so its menu pops up
4) Click elsewhere in the app to close the menu. Don't move the mouse
5) Click the mouse again.

EXPECTED: It depends what you've clicked on.
ACTUAL: The combo box's menu pops up again, even though the mouse cursor is nowhere near it.

This continues to happen until you move the mouse once a pop-up menu has closed. I've attached a small example Qt Creator app that shows this going wrong. There's a custom button and a combo box, both of which invoke menus. You can reproduce the problematic behaviour with either of them.