Titelbar is transparent. How to make it solid again?
This has no effect:
Right click on the task bar and select Show panel config.
Then edit the Panel Settings to get the look you like. Specifically Opacity.
The default titlebar is already opaque. If that’s not what you’re seeing, you’re probably using a 3rd-party one under the “Window Decorations” page in System Settings. If this is the case, the solution would be to switch to a titlebar/window decoration style that’s opaque.