Hoping someone can help. Having problems with any of the pop-out windows eg

Insert > Link > Course Link

Insert > Document > Course Document

The pop-out windows comes out then goes straight back again. When you do it for a second time it stays.

it is most bizarre behaviour and same on different browsers and for different users. We have some custom CSS on the course and wonder if it is linked to this but very unusual behaviour indeed:

Here is a video what is happening - Bizarre pop-out behaviour