"Backward One Second" sometimes unexpectedly goes forward ca. 1 second

OS and Version (please complete the following information):

• Operating System: Debian Trixie
• Shotcut Version: shotcut-linux-x86_64-26.8.1.txz from the project download page

Describe the bug

Backward One Second sometimes unexpectedly goes forward instead of backward.

Steps To Reproduce
To reproduce this issue, please download the project skipright.mlt from:

https://www.metalevel.at/shotcut/skipright.mlt

and the video it uses:

https://www.metalevel.at/shotcut/skipright.mp4

1. Open skipright.mlt in Shotcut
2. Add a key binding (ideally: a single key) that lets you quickly access the function Skip Previous. In my case, I bound this to the number 1 (after removing the default binding of 1, so that the key binding was free)
3. Press End to go to the end of all tracks
4. Our goal is to elicit a case where PgUp (Backward One Second) unexpectedly goes forward instead of backward. We can elicit such a case by quickly alternating between Skip Previous and Backward One Second. In my case, when I quickly press: 1 PgUp 1 PgUp 1 PgUp 1 PgUp .... then it often works exactly as expected, but sometimes during the succession a case occurs where PgUp goes forward approximately one second, instead of backward.

Expected behavior
We expect this key sequence to go strictly monotonically backward in time, never forward.

Thank you a lot for Shotcut! Please let me know any time if you have any questions.

Start by reproducing the issue with skipright.mlt and skipright.mp4 in Shotcut, using the described Skip Previous and Backward One Second key sequence. Trace the handling of those two functions and the rapid alternating input. Done means repeated Skip Previous/PgUp sequences move strictly backward and a regression test covers the case.