With Glib event dispatcher the following sequence of code may assert:

QCoreApplication::processEvents();
Q_ASSERT(QWindowSystemInterface::windowSystemEventsQueued() == 0);

See attachment for compilable test code. It all depends on timing, but it is easier to reproduce when CPU is busy (e.g. when compiling) and mouse hovers over the test window. Sometimes it is enough just to start the test and it immediately (after few iterations in the loop) asserts.