For future reference, two options to set VirtualBox for 4K. I use two windows-only structural analysis applications. Their icons scale very differently:
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SAP - application icons do scale, although they get somewhat smaller at 4K, but still usable.
VM configuration>monitor:
2D and 3D acceleration enabled
scale factor 100% - this means VBox will map 1 host display pixel to 1 guest display pixel, without scaling, i.e., in windowed mode W7 see the max display res of 3840 x 1953 (1200 less the lines used by openSUSE panel)W7 guest:
display scale 180%. this is set at the custom text size (DPI) option (W7 does not give dpi in dpi, but as a percentage)
pointer scale set to normal.Guest welcome/logon screen text & cursor are small, but readable.
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CYPE 3D - icons don’t scale at all, are so tiny at 4K as to be unusable.
VM configuration>monitor:
2D and 3D acceleration enabled
scale factor 180% - this is how much VBox will scale down the 4K display resolution for the guest, i.e., 3840/1.8 = 2133 horizontal, etc. In my VM in windowed mode, W7 see the max display res of 2133 x 1085 (less than 2160/1.8 = 1200 due to the lines used by openSUSE panel)W7 guest:
display scale 100% (same as standard dpi, only W7 does not give dpi in dpi, but as a percentage)
pointer scale set to extra large.Application icons and guest welcome/logon screen text & cursor are normal size.
Some unusual scale settings like 157 (VM or guest) will make the mouse very, very laggy, although the guest monitor is at 60Hz, same as the 4K panel.
Performance does not feel as good as with the first configuration option, but is usable.