Qt Dock Widgets super slow to dock

In Qt 5.15.3 on Windows, dock widgets can now take several seconds to dock compared to 5.14.1. At first you think the widget sometimes fails to dock but then you realize it has a long delay before it does dock (up to 10 seconds!). This can be reproduced using your example: Qt5.15.3\Examples\Qt-5.15.3\widgets\mainwindows\dockwidgets. Try several dock attempts as it is not always the same each time.

It appears that moving the mouse after the mouse move event to dock causes the dock to happen. So, if you release the mouse button but don't move your mouse, then the bug is easy to reproduce.

Way to reproduce:
1. Build dockwidgets example with Qt version 5.15.2 or 5.15.3
2. Run the built application
3. Drag a widget outside the docking area and drop it.
4. Pick the same widget up and drag it above the docking area of the mainwindow
5. Release the mouse button without moving the mouse
6. You should see that widget won't visually dock if the mouse is not moved.