Third party clients are unable to effectively use the current GlyphVector data (all that is exposed in the API) to determine the boundaries of glyph clusters, which is necessary for client layout of text on a path. If we change the GlyphVector data, we might consider reworking it so that it is useful for this purpose.

This might not require new API per se, if we document the output carefully (it hasn't been before). Of course, it's only the fact that it's not documented strictly that allows us to make incompatible changes to it, so perhaps a new API (for example, 'getGlyphClusterIndices') would be better.