The LightDM login screen (greeter) runs before any user session exists, so it ignores your desktop's display configuration: on multi-monitor setups it typically brings up every screen at its EDID-preferred refresh rate (usually ~60 Hz), unrotated, and with a duplicated background.
This Gist installs a small Bash script into LightDM's official greeter-setup-script hook. Right before the greeter appears, it turns off every connected display except the primary one and drives the primary at its maximum detected resolution and refresh rate. If the primary display is not connected, it does nothing and LightDM behaves exactly as stock.
- Hardware-agnostic: Only the primary output's name is declared; resolution and refresh rate are detected on every boot, so it survives monitor upgrades and works with any number of displays.
- Official mechanism: It uses the free
greeter-setup-scripthook, leaving the distribution's owndisplay-setup-scriptuntouc