In the right hand side burger menu there is an entry for 'configure sources' this entry is a radio button but there is nothing radio about it? :O Reproducible: Always
Created attachment 99338 [details] screenshot
I see what boggles you xD Technically it's radio because it's a state shared between this one and the actions on the toolbar. I agree it's not very intuitive, I'll wait for the Kirigami redesign to take action here.
Doesn't apply as of 5.8, let's discuss over the new design.