The way it works for your terminal emulator example is that it figures out what makes sense to do for a value of 1.785, e.g. rasterizing text appropriately and making sure that line heights and baselines are at sensible consistent values.
the problem is that there's no reasonable thing to do when the height of the terminal in pixels is not an integer multiple of the height of the font in pixels. Whatever "it" does, will be wrong.
(And when it's an integer multiple, you don't need scaling at all. You just need a font of that exact size.)
You're overthinking things a bit and are also a bit confused about how font sizes work and what "scaling" means in a windowing system context. You are thinking taking a bunch of pixels and resampling. In the context we're talking about "scaling" means telling the software what it's expected to output and giving it an opportunity to render accordingly.
The way the terminal handles the (literal) edge case you mention is no different from any other time its window size is not a multiple of the line height: It shows empty rows of pixels at the top or bottom.
Fonts are only a "exact size" if they're bitmap-based (and when you scale bitmap fonts you are indeed in for sampling difficulties). More typical is to have a font storing vectors and rasterizing glyphs to to the needed size at runtime.
Right, but most users of terminal emulators typically don't use bitmap fonts anymore and haven't for quite some time (just adding this for general clarity, I'm sure you know it).
