Note: In recent versions of KDE Plasma (5.18+), Latte dock is interfering with the particular setting described in this answer (After every restart, the user has to run the command in this answer once again, because Latte overwrites the configuration file). The workaround is to let Latte dock manage the borderless window configuration. See this answer for more details.

To remove the borders of maximized windows in KDE Plasma 5, run the following commands:

kwriteconfig5 --file ~/.config/kwinrc --group Windows --key BorderlessMaximizedWindows true

qdbus org.kde.KWin /KWin reconfigure

This is equivalent to doing the following:

1. Edit the file ~/.config/kwinrc to add the line:

   
   

   BorderlessMaximizedWindows=true
   
   

   
   

   under the [Windows] section, so that it looks something like this:

   
   

   
   
    [Windows]
   
    BorderlessMaximizedWindows=true
   
    ActiveMouseScreen=true
   
    AltTabStyle=KDE
   
    AutoRaise=false
   
    AutoRaiseInterval=750
   
    BorderSnapZone=10
   
    CenterSnapZone=0

   
   

   In older versions of KDE, the file may be located at:

   ~/[.kde|.kde4|.kdemod4]/share/config/kwinrc

   
   



2. Restart KWin by running:

   
   

    kwin --replace
   
   

   
   

   If that does not work, try one of the following, depending on which display server you are using:

   
   

    kwin_x11 --replace
   
    kwin_wayland --replace
   
   

   
   

Go crazy and maximize every window in sight! Remember that you can unmaximize them from the window menu (Alt + F3) or from the task manager in your panel. If quick tiling is enabled (System Settings → Desktop → Screen Edges → Window Management), you can also use Alt + Left mouse button to unmaximize windows by dragging them.

References

• Hide window border for maximized windows