The main event loop temporarily stops processing events during certain UI interactions, such as opening a menu or dragging the mouse.

For an example, see http://stackoverflow.com/questions/18600452/qmainwindow-stops-receiving-qeventupdaterequest-when-user-opens-menu-or-resize .

I think this is because Qt's main loop is attached to Cocoa's default run loop only, and not to others such as the event-tracking run loop.