I alt-clicked on a window's title bar and removed the bar by clicking "No border" but now I have no accessible means to restore that window's title bar. Is there a way to open KWin for that specific window? What are my options for restoring the window's border? (Plasma 5, if it's version-specific.)
press Alt+F3 on window with focus:
choose menu item(s): more actions > [deselect] 'no borders'
You should get a message dialog when you 'hide border' that reads something like ... blah, blah. use Alt+F3 to show menu allowing you to do the above with your mouse.
If you are Not getting the message do this (utf-8 editor!):
Change or Add the Line(s):
[Notification Messages] altf3warning=true
Save, Done. (no restart necessary)