I have been using the mouse to work around this issue in lxqt in Sparky-testing (Siduction repo for lxqt) for quite some time. I am not dealing with this in siduction lxqt via Indian summer as I am still using lightdm there. This was present for sddm 0.12 and also sddm 0.13 for me.
https://github.com/sddm/sddm/issues/501
Trial usage of the recent lxqt Paint it black with sddm shows this has been fixed for this case. Thank you. I could hunt up how this was done, but if anyone has the short answer I would appreciate. Sddm has it down as something they want to fix in 0.14.