Can't use unscaled HiDPI with Linux guest if display scaling enabled in OS X Yosemite host

[MikeUatCS]

Host is a MacBook Pro running OS X Yosemite 10.10.5, connected to a 3840x2160 display. Guest is Debian 8 with KDE 4.14.2, guest additions installed. I am attempting to run the guest OS in full-screen mode.

Display scaling is enabled in the host, with the scaled resolution set to 2560x1440. If "Use Unscaled HiDPI Output" is UNchecked, the guest correctly sees 2560x1440 as a resolution choice, and defaults to it.

If "Use Unscaled HiDPI Output is checked, then the display does not work properly. The highest resolution reported to the guest appears to be 2560x1600 (not 3840x2160 as it should be), and for some reason the resolution selected by default is 1024x2880 (which isn't even a valid resolution). This results in the entire display being compressed into a narrow vertical strip up the middle of the screen, with everything squeezed to illegibly small size. Manually selecting 2560x1600 in the guest results in a similarly unreadable rectangle in the center of the screen.

Disabling display scaling on the host results in correct guest behavior, allowing guest to access full 3840x2160 native resolution. Unfortunately, this results in UI elements on host OS X desktop being unusably small. So it seems it is impossible to have both a usable host desktop, AND give the guest OS access to the full native display resolution.

To summarize, the functionality of the Use Unscaled HiDPI Output checkbox appears to be broken in 5.0.4, at least for the hardware/software configuration described above.