Legacy applications are somewhat blurry when moved from high-dpi to low-dpi, but anything modern works perfectly well. In my experience, X with current gnome or KDE is completely unusable in the same scenario. Either everything is huge on the low-dpi screen, or tiny on the high-dpi screen.

There is a third option: xrandr lets you choose different resolutions per-screen. Nothing ends up too large or too small, you just don't get the benefit of the high-dpi screen.

This makes it completely pointless to even have a high-dpi screen. I think it is a pretty common use-case to have a new 4K monitor together with older FHD ones, or a high-dpi laptop with a low-dpi external screen. X really shows its age in relying on everything having roughly the same pixel density.

Not completely; I do this while docked and the laptop screen is further away, when high-dpi doesn't matter as much, and go full-res when mobile.

> or a high-dpi laptop with a low-dpi external screen

Yep, that's where I am right now, which is why I wanted to mention the third option - our IT support people upgrading everyone's laptops didn't even know this could be done.

Legacy applications like Microsoft Outlook