I am using Qt 6.8.1 with Buildroot 2025.02.3 on an ARM board running embedded Linux.

When using the framebuffer platform plugin, everything works fine with QComboBox, and I can open the popup menu without any issues. However, when using the VNC platform plugin, the popup menu sometimes fails to open when I click on the combo box. If I then close the editor (which destroys the combo box) after such a failed attempt, the application crashes with a segmentation fault (SEGV_MAPERR).

If the popup menu does not open, I can continue to work normally as long as I don't close the editor. The popup menu often opens on the second attempt. However, if it had failed once, closing the editor afterwards will still cause a crash. The problem is easier to reproduce when the combo box contains more entries (for example, 27 entries). It occurs approximately once every 10 tries.

I could not reproduce this issue on my X11 openSUSE build.

The issue occurs both with the standard QComboBox and with my subclass QMyComboBox. In QMyComboBox, I only overrode showPopup(), hidePopup(), and eventFilter() to add logging (see QMyComboBox.h / logging.txt). The behavior of QComboBox was not modified.

From the logs, I can see that the issue occurs if the mouse release event is processed while I am still in the showPopup() function of the QComboBox. In this case, it calls hidePopup() from within showPopup().

As a workaround, I introduced a guard variable that fixes the issue for me (see QMyComboBox_workaround.h).