Out of curiosity, what is a non-broken behavior supposed to be if the screens are combined? Do people expect the low-dpi screen to be a blurry version of whatever would appear on the high-dpi screen, or do they expect it to act like a magnifier, perhaps with pan/zoom controls?
Those days are gone. No matter how hard you try - either you are simply not runing on current hardware, have very modest expectations or close your eyes to realize that the Linux desktop in this regard is at least two years behind, minimum!
For reference, I have been using Linux continuously on all sorts of hardware since 1994. What I lack is any practical experience using modern Windows or Mac OS, so I have no idea what implicit expectations you may be bringing from those. The last time I ran Windows directly on hardware was before Windows 95 was released, and similarly my only real Mac experience was on monochrome classic Macs before OS X existed.
Over the years, I have used just about every sort of display hardware with Linux, ranging from serial terminals, Hercules monochrome graphics, 800x600 through 1600x1200 CRTs, the first wave of DVI-based LCDs, various HDTVs, the first DLP projectors, and my current dual 4K monitors. I was also involved in the early testing and deployment of 2D and 3D accelerators on Linux, as well as things like clusters driving arrays of projectors. We even had one of those IBM "Big Bertha" displays in our lab at one point, which was one of the first 300 DPI LCD monitors available. Just about the only thing I haven't used with Linux is head-mounted displays nor stereo glasses. My last involvement with VR was 20 years ago when SGI Onyx-based CAVE systems were prevalent in academia, combining head-tracking, active shutters, and multiple wall projectors.
But, to be honest, I have no use case to combine different DPI monitors into a single graphical screen or desktop. If I connect a laptop to a projector or display panel for presentations, I tend to just want to duplicate the presentation view on the internal screen. Otherwise, I use the laptop to be mobile and I use workstations with their dedicated displays.
Like the previous poster, I stick to 1920x1080 on my 14-inch class Thinkpad. In my office I have dual 28-inch 4K monitors. These have identical dot pitch to my Thinkpad, so each monitor is like having a 2x2 array of my Thinkpad screens. When I made sure the monitors were set with the correct DPI, everything worked exactly as I would expect. Whatever rendered as one pixel on my laptop would also be one pixel on my workstation, and I just had 8 times more real estate on my dual monitor desktop. But, I sit further from the monitors than I do my laptop screen while also using them for much longer stretches of time. So, I adjusted the workstation to pretend it had higher DPI so that things would render a little bigger.
It's been a few years, but I think I may have had to separately adjust Firefox because it has some of its own weird assumptions about fonts and DPI that I assume come from their renderer straddling several different platforms. I also had to adjust emacs and xterm to change from my decades old fixed font preferences to start using scalable fonts.