Comment # 52 on bug 1089932 from Michal Srb

(In reply to Fabian Vogt from comment #51)
> As the DPI is not overwritten by starting Xorg with "-dpi", I expect the
> logical and physical DPI to be the same -> reassining to X.

AFAIK this is usual X server behavior. If it is not given the -dpi parameter,
it defaults to 96 dpi for calculating (guessing) the screen physical size. The
reasoning is that the screen physical size is meaningless whenever you have
more than one monitor connected. It is there mostly for backwards
compatibility.

The proper source of information is the physical size of the individual
outputs.

I can imagine that the screen physical size could replicate the output size
when only single output is connected (maybe that's what the nvidia driver
does), but there may be issues when monitors connect/disconnect. Ideally
sddm/Qt should consider only the information from the outputs.