The shortcut to switch between mirroring and extending your display on a Chromebook is the Control key + Display key.
This combination allows you to quickly toggle between having the same image on both your Chromebook screen and an external display (mirroring) or extending your desktop across both screens.
Essentially, pressing Ctrl + Display key will cycle through display modes, including mirroring. The "Display key" is the key on your Chromebook keyboard that looks like a monitor, often found in the top row of keys with function keys.
