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.)
4 Answers
Check in Systemsettings > Application Style > Window Decorations
there should be an option to set borders.
or edit the following config file: ~/.config/kwinrc
look for the line: BorderlessMaximizedWindows=true
and either change it to false or comment it out.

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Neither of these apply to me. I'm also on Plasma 5. There's no option to set borders, and the line in kwinrc already doesn't exist. – chreekat Jun 29 '18 at 13:47
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this setting is non-existent in Plasma 5 – Sajuuk Aug 13 '21 at 13:05
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I can clearly hear the voice of a colleague now, he's always around when there's a problem with linux that requires this kind of solution, while in another OS you just press a button. Luckily I've solved with Janghou's answer...and his voice went away :-D – funder7 Jul 29 '22 at 22:31
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!):
Edit file: ~/.kde4(.kde)/share/config/kwin_dialogsrc
Change or Add the Line(s):
[Notification Messages]
altf3warning=true
Save, Done. (no restart necessary)

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1The first half of your answer duplicates Janghou’s answer. – Scott - Слава Україні Jul 08 '20 at 03:27
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I needed to do the same thing, but I didn't find anything relevant in ~/.config/kwinrc
.
I found the application-specific settings in ~/.config/kwinrulesrc
. I removed them with Vim, and restarted kwin_x11, and restored the border on the program I had previously removed the border from.

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