Bug 400505

SUMMARY
Tasks disappear after resolution changes.

Specifically, I am talking about the place where tasks of the default task bar where applications go when you click on the minimize icon of the window manager. 

STEPS TO REPRODUCE
1. Have a 4K screen configured to use 4K and another screen with a 1920x1080 resolution in which at least one application is displayed on the taskbar of the 4K screen with both physical screens showing an instance of the task manager showing "only tasks of the current screen".
2. Modify resolution configuration for the large screen to also be 1920x1080.


OBSERVED RESULT
Both task managers show no tasks.


EXPECTED RESULT
Tasks would still be shown. Task bar entries and their display should be invariant w.r.t. resolution modifications.


SOFTWARE VERSIONS
KDE Plasma Version: 5.13.4
KDE Frameworks Version: 5.49.0
Qt Version: 5.11.1

ADDITIONAL INFORMATION
Bug 373075 is the same, but my description is more specific. Bug 189337 was closed a long time ago, because it was old, but might be the same. 

RECOMMENDED IMPLEMENTATION
Implement a regression test, which iterates through all combinations of resolutions that are in use by Plasma5 users. Specifically the test should start Chrome, Firefox, Konsole displaying entries on the taskbar first and it should test that after the resolution change the task manager is not empty.

Please run such a regression test suite before each release, since such bugs shouldn't exist for almost a decade. 

RECOMMENDED PROCESS CHANGE
Stop writing new features for which you do not have either exhaustive or tests written in a framework like hypothesis (e.g. https://hypothesis.readthedocs.io/en/latest/). I.e. try to make a release which has *no* bugs.