Only a part of the screen gets mirrored to Apple TV (MacBook Pro with external monitor)

Hi,

I have the following issue. I have a MacBook Pro that is always plugged into an external monitor when it's at home. I rarely use the laptop screen, the MacBook Pro stays closed most of the time. I just bought an Apple TV. When I mirror video from my MacBook Pro to my TV through the Apple TV, it's as if the MacBook Pro was sending to the Apple TV just the part of the image that would be shown if the image I have on my external monitor was seen on the laptop screen, resulting in having on my TV just the top-left corner part of the image that is shown on my external monitor. So, on my external monitor, everything is fine, but on the TV, a big part of the image is missing. When I unplug the external screen from the laptop and open its lid, it solves the problem, but I don't want to have to do this everytime I use my Apple TV. It's quite hard to explain as there are three screens involved here, but I hope you understand.

Am I the only person using a MacBook Pro with an external monitor who tries to mirror that on to an Apple TV? Do you have similar issues? Any workaround?

Thank you very much!

Etienne