Apologies for not knowing the verbiage or providing a pure Qt only example to show this bug. When hold+dragging mouse and flicking it off-screen quickly the pointer changes unpredictably. This can be seen in the following program Scribus built on Qt5 framework:

Screencast: https://www.dropbox.com/s/c5wclmcyzqhazpq/example.avi

How to reproduce:
1) Open Scribus
2) Open new document
3) Press 't' to start a text box
4) As shown in Peter's 2nd clip (https://www.dropbox.com/s/c5wclmcyzqhazpq/example.avi?dl=0 [^]) start the text box but don't close it. Instead start to whip it downward in a quick motion off the canvas and in to the bottom panel.
Keep repeating the motion until you can reproduce the bug. I was able to reproduce it after 4 iterations.

Reference downstream ticket: https://bugs.scribus.net/view.php?id=12798