On Linux, it is becoming better, but still a lot more to improve.
Even on Fedora 26, I have to manually patch and recompile Mutter to get HiDPI support on Wayland. Mutter is hardcoded [1] to use 2x scaling on displays that are 192 PPI or more. My Dell p2415q is 188.2 PPI, so you get 1x 'scaling' by default and everything is tiny. This problem has been known for a while [2].
And even though GNOME applications then generally work well, you have to start Chromium with a special flag to let it run with scaling. There are a lot of glitches throughout the system, e.g. the mouse cursor has the right size, but when you go to a window corner to resize a window, it becomes tiny. A lot of icons are blurry, because they have a low resolution and are scaled up.
macOS is basically plug & play. All applications are in full HiDPI glory. The only exception are websites that use low-resolution images. There is one catch: the (terribly expensive) Apple USB-C Digital AV Multiport Adapter only supports 4k at 30Hz. This refresh rate is very tiring and annoying. However, a USB-C -> DisplayPort ALT mode connector works great @ 60Hz.
[1] IIRC there is work in the master branch to solve this.
Chrome/Chromium is notoriously bad at adopting any technology. On Windows, it took them years to support scaling other than 1x, color Emoji was added very recently, etc. I think on Linux it is even worse. Have they finally fixed ligature support in Chromium?
With Firefox rapidly improving in performance and memory usage, less and less reasons to use Chrome.
I set Xft.dpi: 192 in ~/.Xresources (gnome-settings-daemon and others will perform the equivalent setting).
Chromium respects this (for many major versions by now) and shouldn’t need a separate flag. Are you certain the flag is necessary in your setup? Can you check what Xft.dpi is on your system? xrdb -q | grep '^Xft.dpi'
Even on Fedora 26, I have to manually patch and recompile Mutter to get HiDPI support on Wayland. Mutter is hardcoded [1] to use 2x scaling on displays that are 192 PPI or more. My Dell p2415q is 188.2 PPI, so you get 1x 'scaling' by default and everything is tiny. This problem has been known for a while [2].
And even though GNOME applications then generally work well, you have to start Chromium with a special flag to let it run with scaling. There are a lot of glitches throughout the system, e.g. the mouse cursor has the right size, but when you go to a window corner to resize a window, it becomes tiny. A lot of icons are blurry, because they have a low resolution and are scaled up.
macOS is basically plug & play. All applications are in full HiDPI glory. The only exception are websites that use low-resolution images. There is one catch: the (terribly expensive) Apple USB-C Digital AV Multiport Adapter only supports 4k at 30Hz. This refresh rate is very tiring and annoying. However, a USB-C -> DisplayPort ALT mode connector works great @ 60Hz.
[1] IIRC there is work in the master branch to solve this.
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1258155