How does characterboundsupdate interact with multi-codeunit characters?

The spec doesn't seem to explicitly say that the number of rectangles passed to updateCharacterBounds should equal event.rangeEnd - event.rangeStart, but the example implementation does it that way, and it kind of seems implied by the fact that the browser needs to be able to find the appropriate rectangle for a given character by offset and the rectangles don't get explicitly associated with a specific position, except for their array position.

Since a given 'character' can take up multiple string positions, how should astral characters be handled here? Repeat their position multiple times in the array? If so, that seems non-obvious enough to mention explicitly. (But it also seems like a somewhat awkward solution, and defining this in such a way that the number of rectangles should match the number of actual unicode characters, not code points, between the given offsets, would also be reasonable, assuming the API can garantee that the queried offsets never fall in the middle of a surrogate pair).

Start by reading the specification language around updateCharacterBounds, event.rangeStart, and event.rangeEnd, then compare it with the example implementation mentioned in the issue. Determine how astral characters and surrogate-pair offsets should map to rectangles; the work is done when the spec explicitly defines the required array length and position mapping.