Created attachment 110636 [details]
Comparison of windows shown when clicking and when hovering the mouse
Sometimes, when I have multiple instances of an application running and hover the mouse over the icon of the application in the icons-only task manager, it shows more windows there in the previews than there are windows open, with some of them being duplicates, whereas clicking the icon shows the correct number of windows.
I'm attaching an example screenshot with the windows that both effects (clicking and hovering) show, superposed in one image only as I couldn't attach more than one file. This tends to happen quite often with the Vivaldi browser.
*** This bug has been marked as a duplicate of bug 389469 ***