If I use the _cap feature to add serifs to my /H and my /I, and I have my /I set to borrow its sidebearings from /H, it looks like Glyphs wants to use the serifless contour when taking the measure from /H, but including the serif width when applying it to /I.
I’m not sure if the ideal would be to always include the serifs in assessing the sidebearing, or to never do so. But obviously if it’s inconsistent as here, the shared sidebearings feature is incompatible with the _cap feature.
Historically the serifs are ignored when establishing sidebearings.
Yes, it’s clearly the heavy stem that optically plays the biggest role in eyeing sidebearings, and actually if that’s the direction that Glyphs goes it would make sense. Maybe it’d be unprecedented in digital font editors? But it does mean that if you want to link sidebearings and you make caps for your H and I, you’ll also have to make caps or corners for your B, P, L, etc. (And have to wait for the bug I described in the top post to be fixed.)
I’ll check the metrics keys with caps. That is clearly a bug. And for now it should respect the series.