DPI scaling is way off on KDE/Plasma

This is a bug report related to NVidia proprietary drivers. All the information and comments is here:

DPI scaling is way off when using nvidia proprietary drivers instead of nouveau

Summary: fonts and UI is bigger than should be.

What are the actual physical dimensions of the display? Those size numbers are taken directly from the EDID and the DPI is computed from that.

Screen size is 17.3" FHD.

That isn’t what he wants, post the output for

xdpyinfo |grep -e resol -e dim

$ xdpyinfo |grep -e resol -e dim
  dimensions:    3840x2160 pixels (1016x571 millimeters)
  resolution:    96x96 dots per inch

Here you have it.

$ xdpyinfo |grep -e resol -e dim
  dimensions:    1920x1080 pixels (381x211 millimeters)
  resolution:    128x130 dots per inch

That looks correct for a 17.3" display. Nouveau is likely just returning 96 DPI instead of calculating it from the monitor EDID. See: DPI from the README. You can set it to ignore the EDID and force it to 96 DPI in the xorg.conf/xorg conf.d.

The problem with clipping in the screenshots you show are bugs in those programs. They aren’t handling hidpi displays correctly.

Since the nvidia driver and card firmware are closed source, I cannot know for sure if the issue is on Nvidia’s side, which is from my understanding.