Created attachment 180256 [details]
Video of the issue in play

***
If you're not sure this is actually a bug, instead post about it at https://discuss.kde.org

If you're reporting a crash, attach a backtrace with debug symbols; see https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports

Please remove this comment after reading and before submitting - thanks!
***

SUMMARY
Can't resize a "Synchronous message" UI element between two lifelines

STEPS TO REPRODUCE
1. New squence diagram
2. Create two lifelines
3. Create a new Synchronous message object between the two lifelines
4. It is not possible to extend the length of the Synchronous message box on the receiving lifeline

OBSERVED RESULT
Mouse drag inoperant, the object is moved instead of resized.

EXPECTED RESULT
Widget should be resized in length upon click+drag when hitting the "resize" corner element.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 24.04 via Snap https://snapcraft.io/umbrello or flatpak (same outcome) https://flathub.org/apps/org.kde.umbrello
KDE Plasma Version: 5.15
KDE Frameworks Version: 5.15
Qt Version: 

ADDITIONAL INFORMATION
Looks similar to 120910; possible regression?