When editing the content of a page in "in-place" mode, and the page is long enough that one needs to scroll down to see all the content, the toolbar of the WYSIWYG editor jumps to the bottom of the edit area when scrolling to the bottom of the page. This obscures the "Save" button (and the other buttons).
The obvious workaround is to scroll up a bit to make the buttons visible again.
Or click elsewhere except in the editable area, as pointed out by mflorea.
I can produce this with Firefox 76.0 / Ubuntu 18.04.2