Some issues related to "Respect physical display sizes" commit

I want to open this issue to reference all problems introduced by commit b032e7aa and MR !6 (merged). I think this is cool to have an auto detection instead of hardcoded 96DPI by default, but we saw some side effects after extended tests (and @ochosi too).

I have a laptop (1920x1080) and an external screen 27" (1920x1080)

When I have both pluggued, and starting the session.

xdpyinfo/xrandr :

 screen #0:
   dimensions:    3840x1080 pixels (891x162 millimeters)
   resolution:    109x169 dots per inch

Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 16384 x 16384
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 293mm x 162mm
   1920x1080     60.03*+  48.03  
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 598mm x 336mm
   1920x1080     59.93*+  60.00    50.00    59.94  

• The font size is "109" (based on DPI), and works fine for me.

However, when I start my session without external display, I have huuuge fonts:

1243-screen #0:
1254-  dimensions:    1920x1080 pixels (293x162 millimeters)
1310:  resolution:    166x169 dots per inch

Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 293mm x 162mm
   1920x1080     60.03*+  48.03 

When I run xfsettingds --replace multiple times, I don't always have the same result, but can't have a reproductible way for now.

As a side note:

• Qt apps have huge fonts (like virtualbox or vlc). Running them with QT_FONT_DPI=96 fix the problem.

@gwhitney what do you think ? Did you encountered theses behaviors ?