Welcome to the world that Fedora users have been living in for years already. Ubuntu users can thank to Canonical's decision not to enable Wayland in 18.04 for that...
If it really was intentional, a small dialog or log that explains _why_ I couldn't scale monitors differently would have sufficed, or, simply moving the percentage adjustment control out of the settings for each display.
On that note, 20.04 is supposed to support fractional scaling, but if I try to turn the toggle on, it just fails silently, so are we back to square one, or did a developer somewhere decide some precondition means I can't _currently_ turn it on, but didn't feel it was worth telling me, the user, why?
In the past, when it wasn't exposed in GUI, and worked for Wayland only, it was necessary to logout and login between enabling fractional scaling and setting the scale itself (the reason was, that Xwayland was started in different mode then; technicality, that doesn't matter no normal users, but then, the functionality was labeled experimental and enabled via gsettings anyway).
The X11 fractional scaling is a different beast; it uses xrandr scaling and requires cooperation from the graphic card (the output encoder, specifically - it sets up framebuffer at different size than the output to display). That also means, that it won't work on machines that do not have output encoder at all and assume framebuffer resolution is equal to output resolution, like remote sessions or virtual machine clients. Here I agree, that some error message would be appreciated. It was also brave from Canonical to enable it for LTS, without going through a testing cycle in a non-LTS release.
With a hindsight, the remote desktop/screen sharing solutions caught up faster (except the closed source ones - they have no motivation when ubuntu works, so why bother) than figuring out how to handle several displays with different dpis under X11 so that legacy apps not using modern Gtk/Qt work.
You can do it on pretty much any modern X on linux using xrandr, FYI, it's just not easily done in a settings app. I've been doing this with xfce for a while.
My xrandr line is not that easy to read but effectively, after I've set xfce hidpi support to double the size of screen elements, I increase the view window of the smaller screen by a factor of 1.8, and of the larger screen by a factor of 1.2, and put them next to each other -
xrandr --output HDMI-0 --mode 1920x1200 --pos 0x100 --scale 1.8x1.8 --output DP-2 --mode 3840x2160 --primary --scale 1.2x1.2 --pos 3456x-180
If Wayland can do that without so much messing around, great :)
On X11, scaling of entire framebuffer (i.e. scaling macOS style) is a standard xrandr functionality, but it was never exposed to UI. It has some drawbacks that macOS never had to solve (how it behaves over remote x11/vnc/etc, or memory requirements for framebuffer, for example), that make it unsuitable as a general solution. It is basically one of the hacks complained in this threads.
Are you saying it would have worked had I used xrandr?
Are you sure you were running Wayland? In Fedora under Wayland, you could pick a monitor and choose scaling for it in the setting applet.
It is quite possible that 18.04 has some bugs. I don't have 18.04 machine with hidpi monitor at hand, so I can't have a look myself, so I chalk it up that there is something weird happening.
I'm using the proprietary Nvidia driver.
My XPS 13 is using Intel from an 8th gen i7.
I own a pair of Master Dynamic MW07 and a UE BOOM speaker. As for my laptop, a Dell 7470 with Ubuntu 18.04.4
I've found that without Blueman installed my earbuds will pair but are not even detected as being an audio device. I dislike having Blueman installed/running so I'd like to get it working without.
I'm running Ubuntu 20.04 upgraded from Ubuntu 19.10 on an XPS 13 (it has worked on neither). I'm tempted to test on my Ubuntu 18.04 laptop (XPS 15) to see if perhaps it was broken recently.
One thing of note perhaps, is that the XPS range use the horrid "Killer Wireless" card from the Alienware/Gaming range, their Linux driver performance has been generally horrid in the half a dozen of them I've experienced.
I must therefore conclude that something has been broken since 18.04 as it neither worked for me on 19.10 nor now on 20.04.
It always had the options there in previous versions, but it only ever honoured the value you set on a single monitor. Now, I can have my 4K monitor at 200% while I leave my laptop at 100% (1080p) and it works perfectly.
If only it handled Bluetooth audio devices better it'd be perfect; Currently, I can't connect a pair of truly-wireless earbuds and play audio out of them without using Blueman to mess around with the audio sink settings etc each time I connect them. On my desktop, with Manjaro (KDE) and an Intel AX200 WiFi/Bluetooth I can connect both the earbuds (with no additional tweaking) and for example an Xbox One S Bluetooth controller (with the minor tweak of disabling etrm), and they both work perfectly.
I'm not sure how much of that is related to Gnome/KDE but Ubuntu really could do with some better Bluetooth device handling, especially for audio devices.