Save Online dialog gray after minimizing

If I minimize the Save Online dialog, or switch desktop so that it is no longer visible, then it goes gray when I maximize it again and the controls are no longer visible.

Workaround 1: The problem does not occur when I bring a different application window above the dialog, such as by the using Alt+Tab shortcut to switch between running applications.

Workaround 2: After the problem occurs, right-click on gray dialog > View page source. Right click again > Back.

Regression: This problem did exist in 3.4 but has not always existed (not a recent regression).

I'm using the AppImage
OS: Ubuntu 19.04
Arch.: x86_64
MuseScore version (64-bit): 3.5.0
revision: 43c5553