Eww, looking at this PR again - plz note that offsetWidth & offsetHeight are defined as integer, where getBoundingClientRect gives fractions. Ofc ideally we want the cells to be spaced in integers, but I am not sure if the dimension calculations in the renderers still will yield the same numbers here.
To get more accurate fractional numbers we'd have to measure multiple chars at once (already applied for width in the DOM PR).

Edit:
Still gonna change it in the width cache to match this (as I did not find any glitches from it), but we prolly should address this with a follow-up.