@paulmiller Thanks for the test.
… & all : The HiDPI support is in fact a tweak, the same kind that DT uses, i.e. it force a GDK_SCALE to 1 and manually handle the 2x resize. It works well for Linux and Windows, but not for MacOS it seem. So until we update our HiDPI support, MacOS users who wants to have sharp icons will need to avoid HiDPI resolution as shown here (but does this panel still exisit?) or here (the one with the thunder symbol should be avoided).
@Morgan_Hardwood asked for making this pseudo-HiDPI support optional, which can be done in the Preferences/General panel of the latest dev version (included in future RT5.6rc2). Of course, your icons and preview image will be blurry, like for RT5.5.