Bizarrely just had this happen when trying to change the location of one of my observations. The blue update observations button (and the cancel button) were both visible, then all of a sudden the window seemed to expand and/or shift downwards and they disappeared, with no option to scroll down the page. Shrinking the page didn’t fix it either.
This problem occurred on desktop using Opera. I tested uploading in Chrome, screen was normal. I then went back and opened a new Opera tab, screen was also normal…
So the problem seems to be ‘fixed’, but posting in case this happens to someone else too