Fix Brightness Control on Windows 11

The slider is missing entirely

• On a desktop PC this is normal: Windows does not provide a slider for external monitors. Install a utility instead.
• On a laptop, open Device Manager and check Display adapters and Monitors for warning icons.
• Reinstall the graphics driver from the GPU vendor or the laptop manufacturer.
• Avoid generic drivers delivered by Windows Update where an OEM package exists.

The slider is greyed out or does nothing

1. Open Device Manager and expand Monitors.
2. Right-click Generic PnP Monitor and choose Uninstall device.
3. Restart the PC so Windows re-detects the panel.
4. Reinstall the display driver if the entry does not come back correctly.

Brightness keys stopped working

Function keys depend on the OEM hotkey service. Reinstall the vendor's keyboard, hotkey or system control utility, and check the BIOS for a function-key behaviour setting.

Brightness changes on its own

• Turn off adaptive brightness in Settings > System > Display > Brightness.
• Turn off content adaptive brightness control if listed.
• Check Battery saver, which dims the display when unplugged.
• Disable eco or ambient modes in the monitor's own menu.

External monitors specifically

See fix external monitor brightness on Windows for cable, dock and DDC/CI checks.