The following behavior refers to Adobe InDesign, which I consider the benchmark for sophisticated OpenType feature support:

Whichever glyphs are include in the ‘cpsp’ feature, they will be spaced accordingly (whether or not they are capitals), when the ‘All Caps’ feature is applied, or with ‘caps lock’ depressed when typing.

However, if small cap glyphs are included in the cpsp feature, their spacing won’t be affected when smcp or c2sc is applied.

There is no ‘scsp’ feature.

Therefore, if small caps are to be spaced out, it has to be done by giving them wider sidebearings.

The trick is to have the spacing of the small caps match the spacing of the all-cap setting with cpsp applied.

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But how many fonts have this ‘extra sidebearing’ spacing for small caps, tuned to the amount of extra spacing in the ‘c2sc’ feature?

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As an aside, I very rarely include the cpsp feature in my types.
I think the base setting for an all cap setting should be the sidebearings of the glyphs; then the typographer may track in or out from this benchmark according to the demands of the layout, without having an arbitrary value implemented which is unrelated to that.

Thank you Nick!

/KTKM

The one who answered the question, not the one who was sniping at the other.