You have been subscribed to a public bug:
After I plug in a external monitor to my laptop, and change some display
settings, the screen of the laptop displays abnormal.
A possible reason of this issue is about change resolution to a un-fit
resolution, such as from 1366x768 to 640x480. Or
Public bug reported:
Procedure:
1. Boot up the laptop
2. Login
3. Plug in the external monitor
4. Open Monitor Preferences
5. Change laptop's resolution, from 1366x768 - 640x480, change refresh rate.
6. Press Apply
Expect:
Monitor displays normally corresponding to new resolution and refresh
