I installed both QGIS and Qt Designer using flatpak on OpenSuse Leap 15. I tried installing QGIS before Qt Designer or after, but in both cases the QGIS widgets do not show in Qt Designer.
Qt Designer is supposed to find the plugins, and did indeed find many but not the QGIS
Is this a problem with other Linux distros?
Is there a way to tell Qt Designer to fetch the widgets from QGIS?