Steps to reproduce:

• Run qtbase/examples/widgets/mainwindows/mainwindow
• Right-click in a dock widget and hold the right mouse button, to show the context menu
• Continue holding the right mouse button and move the mouse down to the "Split horizontally into" submenu
• The submenu appears then immediately disappears

Screen recording attached.

My expectation is that the submenu remains visible until the right mouse button is released.

If you click and release the right mouse button, then move the mouse, the problem does not occur — the problem only occurs if you drag with the right mouse button, which is a common way to invoke a context menu item on Mac OS X.

Affects Qt 5.6.0 on Mac OS 10.11.4 and Qt 5.12.1 on macOS 10.14.4. Does not affect 5.3.1 on Mac OS 10.11.4. (Maybe this is related to the changes introduced to fix https://bugreports.qt.io/browse/QTBUG-31937 ?) I haven't tested on other platforms.