IIRC it's not that monospaced fonts have double-width emoji; monospaced fonts just tend to not define glyphs for emoji at all, and instead allow the renderer to

IIRC it's not that monospaced fonts have double-width emoji; monospaced fonts just tend to not define glyphs for emoji at all, and instead allow the renderer to fall back for rendering of those glyphs to a non-monospaced font in the stack. This may result in emoji taking up basically-arbitrary width in text edit fields, breaking the monospacing. Terminal emulators, however, have a model on which glyphs are always placed onto a visual grid (the PTY render-model grid, emulating text-mode VRAM); so in a terminal emulator that supports rendering Unicode at all, any glyph will have its visual width "snapped" to an integer multiple of the grid spacing, and will be considered to take up that many grid cells. It is up to the particular terminal emulator where it places the glyph within that array of grid-cells; some left-align glyphs, others center-align.