Undo "No border" in KDE

Focus window:

Alt+F3 ⇒ "more actions" ⇒ uncheck "no border"

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.