[arch-general] xrandr with XPS 13" (3840x2160) HiDPI and 30" (2560x1600) LowDPI

[Tyler]

So,

Using:

Xft.dpi: 220
GDK_SCALE=2
GDK_DPI_SCALE=0.5
QT_AUTO_SCREEN_SCALE_FACTOR=1

xrandr --output DP1 --scale 2x2 --auto --pos 0x0 --primary \
       --output eDP1 --scale 1x1 --mode 3840x2160 --pos 5120x0

I noticed that Qt5 apps look now fine on my external screen (Low DPI)
but now super huge on my laptop screen HiDPI.

I think the reason I avoided using QT_AUTO_SCREEN_SCALE_FACTOR and set
that to 0 was because it was being scaled twice (hence the super
hugeness on my laptop) as apparently it looks at the DPI in xrdb, but I
need that for rxvt, i3 etc.

Without doing what you've done and dropping the resolution on my HiDPI
screen. I found this, using:

GDK_SCALE=2
GDK_DPI_SCALE=0.5

QT_AUTO_SCREEN_SCALE_FACTOR=0
QT_SCREEN_SCALE_FACTORS=2

Single Screen Laptop:

xrandr --output eDP1 --scale 1x1 --mode 3840x2160 \
       --output DP1 --off

All good.

Single Screen External:

xrandr --output eDP1 --off \
       --output DP1 --scale 2x2 --mode 2560x1600

All good.

Both screens!

xrandr --output eDP1 --scale 1x1 --mode 3840x2160 --pos 5120x0 --primary \
       --output DP1 --scale 2x2 --mode 2560x1600 --pos 0x0

Small fonts on external screen with Qt5.

xrandr --output eDP1 --scale 1x1 --mode 3840x2160 --pos 5120x0 \
       --output DP1 --scale 2x2 --mode 2560x1600 --pos 0x0 --primary

Small fonts on laptop using Qt5. So it seems tied to Primary switch as
to which screen it scales for. Fortunately when using dual monitors I
basically never use virtualbox, wireshark or nvim-qt on the laptop screen.

Incidentally if no primary switch is used the eDP1 becomes primary.

-- 
Tyler