Recently I thought I was having problems with Plasma or KDE, but I found the problems happened because I'm using external monitor. The beginning of the problem can be seen in this topic.
https://forum.kaosx.us/d/1636-bug-plasmashell-crashes-on-startup

Use a notebook with external monitor connected to VGA, because the LCD screen has deadline problems. This LCD screen is physically removed, but for some reason it is still detected. In the display settings I just let the VGA enabled.

The question is, is there a way to only force the activation of the external monitor via VGA and disable the LCD notebook?

The simplest would be to disable the LCD in the BIOS, but hé this option in the settings. Physically removed the screen and the cable, but had no effect. The VGA screen appears only when the system starts. This is very annoying, because to access the BIOS I need to connect the LCD broken. The function keys for the second monitor will not work before the loading system.

Is there any way that is configured on the boot and take effect before the start SDDM? Or permanently disable the LCD by the kernel?

I set up "video=DSI-1:d", as the output of the command in Grub, but had no effect

One question in this, does it work correct in GRUB? Or then too the wrong monitor in use?

I did it! Here is a tip I found on another forum:
http://superuser.com/questions/626482/permanently-disable-second-screen

Create a file /usr/share/X11/xorg.conf.d/50-disable-lvds.conf and write there:

Section "Monitor"
        Identifier      "lvds monitor"
        Option  "ignore"        "true"
EndSection
Section "Device"
        Identifier      "onboard"
        Option  "Monitor-LVDS1" "lvds monitor"
EndSection

Thus the system completely ignores the internal monitor.

It is enough for time, but I wish it was able to do it physically, anyway ..