Thanks everyone for the help.  I think this is the right patch, but as with many IDLE patches, it is hard to be sure until it is in use.  At least there is no question here of tk Windows-Linux-MacOS differences.  I checked that the Win 7 buildbots are OK with the PR.

Serhiy: My interpretation of the tk scaling doc is that 1, not 2, is the right argument.  But we can test to be sure.  I meant 'spare monitor', not 'space monitor'.  But I need to find one with a substantially different DPI.

Steve: changing the binaries' manifests is a separate issue, not limited to IDLE or even tkinter.  And 3.8 enhancements do nothing for IDLE on 3.6 and 3.7.  Ditto for a new WinAPI.