I'm working on the crostini terminal where we want all chars to have the same width. We get issues where any characters that use a fallback font (either from the list of css fonts, or the system monospace font) can be a different width to the primary font. This often happens when people use exotic chars in the PS1 prompt, and it is quite noticeable.
I recently landed crrev.com/c/2227711
to fix how we render .notdef tofu so that it renders in the primary font rather than the last resort font. This fixed the width issue for the case where none of the fallback list include a char. But is there any way currently to get fallback fonts to scale to match the primary font width and height, or is this something we could add?