I am not arguing, I am genuinely wondering what you guys want. I have read multiple assertions that Linux is broken but no clear explanation of what it should be doing differently.
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.