My own update: actually XFCE 4.14 does solve the problem. 4.14 did install via my hacky attempt to do it in my W15 (at least most of it did), and its newly included “Window Scaling” setting does the trick.
See it at bottom of this page: https://docs.xfce.org/xfce/xfce4-settings/appearance
I have a HiDPI display (very common now), and for some reason Whonix 15 with 200% VirtualBox scaling is way more blurry compared to Whonix 14.
This is one of the many reasons I’m excited about XFCE 4.14.
But it’s really bad to have a regression from W14 to W15 for HiDPI users. I hate it when Linux does that.
So having said that, @Patrick, what do you think about trying to incorporate XFCE 4.14 into the Whonix 15 OVA, now that it’s in sid (which, I read, is considered quite ‘stable’ anyway)? Just pin Buster repo as the priority but bring the benefits of 4.14 after 4 years of waiting?
After years of crappy blurriness, Whonix could now look as crisp as a native OS. Do we really have to wait 2 more years for that?..