automargin sometimes not recording the sizes

[archmoj]

Overall this looks good, but I'm seeing some odd behaviour with a figure like this one: https://codepen.io/nicolaskruchten/pen/wvWBmyQ

here the long x label is causing some automargin edge case: the right margin is increased because of the slanted label, but the bottom margin would exceed the cutoff so it resets. This is a known problem with automargins, and we don't need to tackle it now BUT the computed margin still shows r: 80 for me, which is incorrect in this case.

Originally posted by @nicolaskruchten in #5203 (comment)

[archmoj]

Also see this minimal codepen.