QPushButtons inside a QTabWidget don't respond to first click after tab-widget change

To reproduce:

1. On a Mac with Qt 5.15.0-beta1 installed, compile and run the example app in $QTDIR/examples/widgets/itemsviews/addressbook

2. When the address book GUI appears, click on the "XYZ" tab at the top right corner of the window.

3. Then click on the "Address Book" tab at the top left corner of the window

4. Then click on the "Add" button at the bottom center of the window.  Be sure to click the button close to its right edge, rather than directly in its center (but still click within the button).

Note that the "Add" button doesn't register the click (if it does register the click, quit the address book app and repeat steps 2-4 again.  For me the fault is reproduced about 75% of the time).  Expected behavior is that the button should turn blue when you click the mouse button down on it, and when you release the mouse button, a sub-dialog should appear.

You can eventually get the button to respond by clicking on it additional times, but it should respond on every click, not just on latter clicks.

This bug is not reproduced in Qt 5.13.0; I think it must be a recent regression.