Check your CSS styles, specifically for the body and html tags. At one point we set the min-height to "100vh" and it caused the issue. You may have a similar style that causes it.
I've seen 100VH issues before in the editor interface in CMS 11 as well. Our solution was to also ship a CMS Stylesheet that was added just for editor mode that overrode 100VH with a fixed height in the CSS.
Since we upgraded to Episerver 12.xx an error in OPE appear.
Empty content areas infinite expanding in on page edit.
We don't have this issue in Episerver 11.xx.
Can anyone think about what might have changed that cause this issue?