[LightDM] Screen flashed after lock with 'dm-tool lock'

[Hacksign]

lightdm + lightdm-gtk-greeter

It is OK if I start computer normally.
But  when I locked screen with 'dm-tool lock',and light the screen 
again.My screen just flashes.
I can see the login panel moves very fast from right to left on my screen.
I can input my passwd, and login my system, but the screen just blacked 
after I succfully logined in.
I had to use 'xrandr --output eDP1 --off && xrandr --output --auto' to 
light my screen again.
here is an image when the screen flashes:

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