What code sequences should we input, and what support should fonts supply? I think spacing and non-spacing marks may need to be addressed separately. For example, manual line-breaking may separate a visually following mark from its base letter, so what has the responsibility of not inserting white space for the typesetter's <NBSP, ZWJ, mark>? (One may be trying to reproduce the line-breaking and phrase boundaries of a manuscript written scriptio continua.) There may be similar problems with preceding marks if the trailing edges of the lines are to be set flush.
Line breaking can also apply to the holes used to string a document together. e.g. the holes punched in palm-leaf documents.