Bug: Buttons disappear when doing browser stuff

The buttons for replying and posting disappear on mundane browser operations such as resizing the window, switching to another tab or downloading content (e.g. an image a user posted) from another tab while the reply input field is open. Seems like some script is running amok, in particular when there are two or more instances of forum pages open at any given time, but I'm too lazy to dig into the debug console. Windows 10, latest version of Chrome (77.0.3865.90, 64 bit).

Mylenium