[macOS][Reg 5.12.4->5.12.5] Popup windows are incorrectly shown when a modal dialog is displayed

When a modal dialog is executed, popup windows are sometimes shown when they should not be. You can use the test application to reproduce. Click the menu button and select one of the menu items. You can also type something into the line edit (eg. "al") which has a completer attached and then select one of the completion options. If you then click the push button to exec a modal dialog, you'll notice that the popup windows associated with the QMenu and the completer are shown. This shouldn't happen.

The attached Qt_5_12_5_popupsIncorrectlyShown.png image shows the two popup windows that should not be displayed marked with purple arrows. The green arrow marks the modal QDialog, which is properly displayed.

Qt_5_12_4_correct.png shows the results using Qt 5.12.4, in which this bug does not appear.

If I just show the tool button menu and/or completion window but do not select a menu item or a completion suggestion, the popup windows are not shown when I later click the button to show the modal dialog. So there's something about selecting an item in the popup that makes it reappear later.

I can reproduce this using a self-compiled build of 5.12.5 from qtbase commit a22bb694ce2. I cannot reproduce the problem using the official Qt 5.12.4 release.

Tested on macOS 10.14 with Xcode 10.2.

[Later]

I updated qtbase to the current tip of the 5.12 branch (commit 53a6f7b7836) and I still see the problem.