Applicaton blocked forever if Drag-Drop in progress while modal dialog opens

If a modal dialog opens while a drag operation is in progress then the application is blocked and in some cases the application must be killed.

The attached source code shows this effect: A QMainWindow is opened which containes a QListWidget and a toolbar-button labelled„Dialog“. If this button is pressed then a timer starts which openes a QMessageBox after 10 seconds.

Please start this program, click the button and then drag one or more of the list items around. Keep them at the mouse until the dialog opens. The effect depends on the operating system and the Qt version in a way that it seems the issue has been fixed for linux but made worse for Windows:

1. Debian Jessie, gcc 4.8.4, Qt 5.6.0:

The drag operation blocks. The dialog blocks as well. Hitting the ESC button leads to a drop operation at a random place. Then the dialog can be closed.

2. Debian Buster, gcc 8.3.0, Qt 5.12.4 LTS

The drag operation is aborted. (OK!)

 3. Windows 7, MSVC 14.0, Qt 5.6.0

The drag operation blocks. The dialog blocks as well. Hitting the ESC button cancels both – in this simple example. In more complex scenarios even ESC doesn’t help always.

4. Windows 7, MSVC 14.0, Qt 5.12.4 LTS

The application is blocked and nothing can be done except killing it via Task Manager.