Since INVISIBLE LETTER is spacing, wouldn't it make more sense to define
Isn't rather INVISIBLE LETTER *non-spacing* (zero-width minimum), even though it is *not combining* ?
I mean here that its width would be zero unless a visible diacritic expands it. It is then distinct from other whitespaces which have a non-zero minimum width, but still expand too with a diacritic above them (width expansion is normally part of the job for the renderer or positioning/ligating tables of characters in fonts).
I would expect that an INVISIBLE LETTER not followed by any diacritic will *really* be invisible, and will not alter the positioning of subsequent base characters (and would not even prevent their kerning into the previous base letter such as in <CAPITAL LETTER V, INVISIBLE LETTER, CAPITAL LETTER A>, where A can still kern onto the baseline below V.
