Than why doesn't it happen when i close Cosmos? Can you explain this?
Do you have Cosmos open on different monitor than 3ds Max? Do you have different scaling on both monitors?
If you take any 3ds Max dialog (for example "Render Setup") and move it to another monitor with different scaling, you will get the same issue.
I am guessing both 3ds Max and Cosmos use the same UI framework and it glitches in the same way.
I just tried it with the render setup on the second monitor and no lagging occurs. I do have different scaling (that is the issue we found before). I am talking about the lagging here, not the pulldown menu's having different scaling.