"No Border" is a very useful function in KWin but by default it does not have any shortcut at all so I suggest that Kubuntu adopt a shortcut for it.
I suggest Meta+H to toggle the function. (thanks to @darinmiller for the alternate shortcut suggestion)
If you aren't aware, this function allows you to remove the Window Decorations of the Active Window.
You can invoke this via the Window Context Menu by doing the following:
- Alt+F3 (annoying default shortcut, should be changed too) -> More Actions -> No Border