Created attachment 111099 [details]
Two Dolphin instances, three windows in the tooltip
Steps to reproduce:
1. Use an Icons-Only Task Manager
2. Open any non-running app
4. Start a new instance using any method
5. Mouse over the app's icon in your Icons-Only Task Manager
The tooltip shows three windows, not two. See attached screenshot. One of the windows in the tooltip doesn't do anything (the other two correctly switch bwteeen Dolphin windows).
If you open a third instance, then the tooltip shows four windows, and so on. It's quite reproducible.
I can reproduce this with Dolphin, Kate, Firefox, Konsole, and any other app that supports having multiple instances running at the same time.
Created attachment 111100 [details]
Two Kate instances, three windows in the tooltip
Created attachment 111101 [details]
Four Firefox instances, five windows in the tooltip
*** This bug has been marked as a duplicate of bug 389469 ***