The panning option might be there as a workaround for a bug in xorg. Since xorg 1.20 the patch to resolve that should be included, so you can try your configuration without panning. https://wiki.archlinux.org/index.php/HiDPI#Side_display The closest I have been able to get was with
xrandr --output eDP1 --auto --output DP1 --auto --scale 2x2 --right-of eDP1 Now everything appears correctly except my laptop screen is on the wrong side. https://i.imgur.com/EEJuNij.jpg When I tried the reverse xrandr --output eDP1 --auto --output DP1 --auto --scale 2x2 --left-of eDP1 I didn't just get a flipped result, I got my browser showing over the middle https://i.imgur.com/QW0IHYw.jpg also the black space in the first screenshot is supposed to be there, that's where there is no screen space as my laptop has a higher resolution than my external monitor. It should look like screenshot 1 except that workspace 1 should be on the opposite side. I also noticed a problem where applications opened on my laptop screen don't have correct scaling on my external monitor which is another thing he spoke of in that blog article.
Second, switching back and forth between dual monitors to one monitor lead to interesting behaviour. For example, I kept losing the scale setting on the lower DPI monitor. exactly that, although it didn't seem to effect GTK3 applications, only Qt5 ones.
Thankfully things in Wayland seem a lot simpler! In Sway it's as simple as https://github.com/swaywm/sway/wiki#display-configuration Have to wait for firefox and thunderbird to have Wayland patches before I switch as the fonts were all blurry. For some reason https://aur.archlinux.org/packages/firefox-wayland/ is really out of date. This has been a major pain for me. -- Tyler