mjhjmtu

The default vertical spacing for fractions accommodates the possibility of a character with a descender or an ascender (or the possibility of subscripts/superscripts). This is, for the most part, the desirable functionality. For example, in the code below,

$$fracga=fracac=fracab$$

nothing needs to be changed. However, there is occasion where an expression like the ones below

$$fracac$$
$$fracab$$
$$fracba$$

occurs in isolation where a reduction in the vertical space on either side, or both, seems warranted to balance the fraction.

If I'm going on a final pass through a document, what would be the best approach to remove this vertical space that would be compatible with a change of font?

I'm not sure it's a good idea. Anyhow, here's an implementation that should only be used in displays (fractions in text style or lower are already adjusted).

documentclassarticle
usepackageamsmath

newcommandafrac[2]%
 dfracadjustbot#1
 adjusttop#2%


makeatletter
newcommandadjustbot[1]%
 begingroup
 settodepthdimen@$textstyle y$%
 raiseboxdimexprdepth-dimen@$m@thtextstyle#1$%
 endgroup

newcommandadjusttop[1]%
 begingroup
 settoheightdimen@$textstyle l$%
 raiseboxdimexprdimen@-height$m@thtextstyle#1$%
 endgroup

makeatother

begindocument

[
fracabafracabquad
fracacafracacquad
fracbaafracbaquad
fracgaafracga
]
enddocument

documentclassarticle
newcommandmyfrac[2]
fracraisebox-.4ex$#1$raisebox.4ex$#2$
begindocument
$$fracpd=myfracac$$
$$fracpd=myfracab$$
$$fracpd=myfracba$$
enddocument