Now I am poking around in the code in 7.6.1 where the issue began when you change the entire file attachment area
The issue has been in all versions of UBB.threads file/attachment manager from 7.6.2 and prior.
It wasnt as apparent in 7.5.x because the Attachment Manager window wasnt integrated in to a modern lightbox. The Attachment Manager window was a re-sizable window outside of the current browser tab.
With UBB.threads 7.6.x, the Attachment Manager window was updated to use modern standards, to be mobile friendly. The constantly reappearing and disappearing scroll bar is what causes the change in size of the window.
This was entirely why I referenced OSx Lion above. Research returns results to say that Opera on OSx Lion is wonky with some overflow-y properties. example: https://css-tricks.com/snippets/css/force-vertical-scrollbar/