Yes, I know: When we talk about Windows, we should avoid physical units -- meters or inches. And when we ask Windows for display DPI, we in fact ask for display scaling percentage. The result 96 "DPI" represents 100 % GUI scaling.

Instead of having just a hidpi flag, how about having a display zoom percentage instead, i.e. 100% for default DPI level and 125, 150, 200 for greater zoom levels, based on what DPI Windows reports. This would allow better control of GUI scaling whenever it is possible, like it is when text is rendered. (And we could drop the term DPI while it is misleading with its false reference to real inches.)

AND: What exactly should be used as the size reference for GUI scaling anyway? Is it the StdFontSize derived from Windows or is it the above zoom percentage derived from monitor DPI, or some strange combination thereof?

Any which way it is, I would prefer having the framework supporting any scaling levels with fine granularity. While icons may cleanly only be scaled to 1x, 2x, 3x, etc., many other aspects of GUI can benefit from greater granularity.