Are you the Elisha Paine that suggested the original fix?

I am still interested in the subject, but there are a lot of factors to consider.

Rereading #33656, I disagree that IDLE is lying; tk scales on startup and others agreed with the current setting, and even thought it should be made part of the Python install.

I did a *lot* of testing, using a magnifier looking at IDLE, Firefox, and Command Prompt, and changing the properties settings.

The minimum client for GetDpiForWindow is Windows 10, version 1607, whereas the current solution works for all Win 10 and likely Win 8, so it cannot replace the latter.

Currently, when I change Settings / Display / Scale and layout, which is what I presume you mean by 'zoom factor' (100%, 125%, ...), IDLE 3.9 changes too, as I would expect.  The text window change is not exactly in proportion to the Display dialog change, but the difference might be the layout white space.  This is contrary to what you report.  I have Win 10 2004 with Sept updates.

When I downscale the resolution, the type for both the dialog and IDLE enlarges.  I presume that this is to keep it smooth rather than becoming pixelated.

I have access to system with 2 4k monitors.  When I downscaled one, and moved Windows between them, the physical height was sometimes maintained, but sometimes changed.  I will probably install Python on that system to see how IDLE and tkinter and turtledemo (without the fix) look, and repeat the above experiment.

If people changes font face, they may need to adjust the 'size', and the physical size for a given 'size' varies between fonts.