Despite having QT_SCREEN_SCALE_FACTORS=2 set, plasmashell does not respect this for some reason. See the two attached screenshots comparing dolphin and plasmashell context menu padding.
Created attachment 101996 [details] bad-context-menu-padding-in-plasmashell.png
Created attachment 101997 [details] good-context-menu-padding-in-dolphin.png
This is similar to bug #372012, which involves kwin.
Unfortuantely enabling it breaks others things worse. *** This bug has been marked as a duplicate of bug 356446 ***
*** This bug has been marked as a duplicate of bug 366088 ***