I'm updated fully as of today under 20.04 and have the same issue described in OP with an NVIDIA M2000M in a Thinkpad P50 using the proprietary drivers and 1 1080p monitor (the laptop panel) and 1 4k monitor (Dell external).
#83 indicated that they changed things so that their PRIME settings were "on demand" and that made things work for them, but my external monitor just quits working when I change it to that.
Things work mostly as expected with the nouveau driver, but no scaling really works under the proprietary driver. I would use the nouveau driver but it seems very sluggish under any scaling factor.
That said, my issue isn't really about fractional scaling, FWIW. I want to set the external display at 200% and set the internal one at 100%, a configuration which doesn't require fractional. But doing this produces 200% on both panels reliably (and the percentage values in settings reflect that both are set to 200%) under the proprietary drivers.
So right now I can effectively either choose from these three options: a) use nouveau, make both displays work as I would like, but have a very sluggish UI experience b) use proprietary and have text that's too small to read for my old eyes on the 4K external monitor but have a usable built-in display, or b) use proprietary to get reasonably sized text and icons on the 4K external monitor but accept that the built-in monitor is not very useful because its effective resolution is 960x540.
It seems that some people have gotten the combination of proprietary NVIDIA drivers and scaling to work. I would try this on another system to doublecheck but I don't have a machine handy with NVIDIA graphics different than the one in my system (M2000M).
I'm updated fully as of today under 20.04 and have the same issue described in OP with an NVIDIA M2000M in a Thinkpad P50 using the proprietary drivers and 1 1080p monitor (the laptop panel) and 1 4k monitor (Dell external).
#83 indicated that they changed things so that their PRIME settings were "on demand" and that made things work for them, but my external monitor just quits working when I change it to that.
Things work mostly as expected with the nouveau driver, but no scaling really works under the proprietary driver. I would use the nouveau driver but it seems very sluggish under any scaling factor.
That said, my issue isn't really about fractional scaling, FWIW. I want to set the external display at 200% and set the internal one at 100%, a configuration which doesn't require fractional. But doing this produces 200% on both panels reliably (and the percentage values in settings reflect that both are set to 200%) under the proprietary drivers.
So right now I can effectively either choose from these three options: a) use nouveau, make both displays work as I would like, but have a very sluggish UI experience b) use proprietary and have text that's too small to read for my old eyes on the 4K external monitor but have a usable built-in display, or b) use proprietary to get reasonably sized text and icons on the 4K external monitor but accept that the built-in monitor is not very useful because its effective resolution is 960x540.
It seems that some people have gotten the combination of proprietary NVIDIA drivers and scaling to work. I would try this on another system to doublecheck but I don't have a machine handy with NVIDIA graphics different than the one in my system (M2000M).