It's not exactly "hidden" or it is too big, it just positions itself at the corner for some reason.
Tried disabling all extensions/ad-block/etc.
Opera GX Version: LVL 2 (core: 71.0.3770.456)
Can happen on any page, kind of random. (Refreshing doesn't help)
(To me it happened on milanote.com (mood board image upload attempt) )
GIF (by OP) above is great, explains everything
Desc: Yeah, basically the pop up appears at the top left corner
My theory is that opera loses track of the upload input element
Steps to reproduce:
Maybe try to create a moodboard on milanote.com
Click to upload an image
Pop up appears near cursor/upload button
Window appears at the top left corner