[HarfBuzz] Can I only reshape broken clusters?

[Kelvin Ma]

If I have the string A = 'affifibcd' and the master list of shaped glyphs

[a 0] [ffi 1] [fi 4] [b 6] [c 7] [d 8]

and I wanted to shape the substring A[2:] ('fifibcd'), could I simply take
my preshaped glyphs after the cluster at or above (ceiling) i = 2 and only
reshape the segment A[2:6]?

So I would reuse the glyphs

[fi 4] [b 6] [c 7] [d 8]

and reshape the segment A[2:6] = 'fifi' to get

[fi 2] [fi 4]

and add them together (dropping the last glyph in the reshaped segment) to
get

[fi 2]  [fi 4] [b 6] [c 7] [d 8]

?
Would this be the same result as reshaping the entire segment A[2:]? Would
this still work for RTL text, or a substring that’s on the left-hand side
of the parent string like A[:5] = 'affif'?
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