Currently, the Window List applet will show an empty icon when focused on the desktop, meaning the button becomes invisible when it's set to only show icons (or on a vertical panel). It also doesn't show any icon if an application refers to an invalid icon (eg. An icon that can't be found on the disk).
Before:
| Window list on the desktop | Panel (Plasma Desktop) | Panel (App with invalid icon) | Icon-only mode with invalid icon or when focused on the Desktop |
|---|---|---|---|
 |
 |
 |
 |
After:
| Window list on the desktop | Panel (Plasma Desktop) | Panel (App with invalid icon) | Icon-only mode with invalid icon or when focused on the Desktop |
|---|---|---|---|
 |
 |
 |
 
|
Btw, there's no fallback icon on the window list itself that appears when you click the windowlist button on the panel. I think taskmanager provides the icons there for all task switchers, so changing that is outside the scope of this merge request.