I just mucked about with Cura and tracked down inaccessible buttons to a lacking implicitWidth for the background of ActionButton in the QML. However, when digging for more information, I found
https://github.com/Ultimaker/Cura/issues/5488#issuecomment-588560291 This suggests setting QT_QUICK_CONTROLS_STYLE=material, and in my testing that does indeed resolve several of the UI bugs, including the crash when previewing and the misrendered (size, placement, missing background) buttons. I shall see if I can set this in Cura itself, but the quick workaround would seem a wrapper script that sets the environment variable. Such tweaks are in fact the main content of the cura script itself (aka cura_app.py), so something like: os.environ["QT_QUICK_CONTROLS_STYLE"] = "material" before the CuraApplication creation (next to last line) should do.
