External display connection issues

[bxskxts]

Recently the external display in my mac stopped working as it should be. When it's set to "mirror displays" it works fine, but when I set it so that the second monitor is an extension (so I can have different stuff on it) the builtin display goes black and just doesnt work at all. Why is that? I tried other cables and adapters btw. Macbook pro mid 2012 non retina, using hdmi cable with hdmi to mini display port adapter

also I can still see my mouse on the default display, there's just nothing to click lol

 

[Fishrrman]

What version of the OS?
What kind of display?
What kind of inputs does the display have?

 

[bxskxts]

OS is high sierra, but the problem was present on sierra too
The display is a TV, it has 3 hdmi inputs