Created attachment 137123 [details] Screencast SUMMARY Minimizing a window resets its thumbnail to the icon of the application. Feel free to close if this is intended, e.g. to prevent redraw cycles. Similarly, invoking the keyboard shortcut to "MinimizeAll" [sic, no space between the two words in System Settings -> Shortcuts -> Kwin"], resets the window thumbnails to the icons of the applications. STEPS TO REPRODUCE 1. Mouse over panel buttons and notice the rendered thumbnails 2. Invoke MinimizeAll 3. Mouse over again OBSERVED RESULT Thumbnails are reset to the icons of the applications. EXPECTED RESULT Thumbnails stay rendered.
You want System Settings > Display & Monitor > Compositing > Keep window Thumbnails: Always. When you tell it to do this, it will display a message indicating the trade-offs that prevent this from being the default setting. :)