Here’s a bit of a puzzle. When the user enters “a/b” in LaTeX mode and formats it with the Enter key, the ‘/’ is marked as “nobuildup”. If the user then switches to UnicodeMath input mode and enters a space after the linear fraction containing the ‘/’, the fraction won’t build up, by design. If you delete the / and reenter it, it’ll build up as usual in UnicodeMath mode. A problem is that the user cannot easily detect that the ‘/’ has the nobuildup attribute since the math ribbon doesn’t display that attribute.

One might conclude that when switching from LaTeX to UnicodeMath input mode, Word should remove the nobuildup effect from all ‘/’ in the current math zone. But the intent in LaTeX is not to build up a fraction with a ‘/’ since built-up fractions are entered in TeX and LaTeX using the special constructs \over and \frac, respectively. The only way the user can build up a nobuildup ‘/’ is to delete the ‘/’ and reenter it. ‘/’ is the only operator that’s marked as nobuildup automatically in LaTeX input mode. A ‘/’ that’s not marked as nobuildup in LaTeX mode is actually used in building up the TeX {<numerator>\over <denominator>} construct. The build-up engine supports TeX as well as LaTeX constructs, since users might use either.

In UnicodeMath input mode, you can mark an operator as “nobuildup” by preceding it with a \. So “a\/b” produces 𝑎/𝑏 and you can try to build it up with a space, but, by design, it won’t build up. It’s fairly common to want to have a simple linear fraction and that’s how it’s done. You can “quote” other operators to prevent them from building up. For example, you might want to quote delimiters, e.g., \{ and \}, which won’t then build up to fit their content.

Perhaps the math ribbons should display the “nobuildup” attribute. Then the user could see a difference. It’d also be handy for the math ribbon to display the bold and italic attributes, since these are commonly used in math zones for math-bold and math-italic characters.