Some fonts may have required ligatures or required ligature like effects. These can be specified through 'rlig', 'ccmp', or (ab)use of requiredFeatureIndex. This is done when the font designer really intended to display the given combination of codepoints in a particular way. The issue is when these required ligatures cross a space, hyphenation point, or other potential line breaking opportunity at the codepoint level. In this case it is preferred not to break the ligature if at all possible. 

This might be doable with the shaper knowing the list of required features and then marking clusters which are tainted by required features as 'unbreakable'. Alternatively, the set of potential line breaks could be handed to the shaper, which could veto, move, or deprioritize as needed (which is approximately what graphite does). With this information users of the shaper can attempt to ignore line breaks in such clusters.

The current behavior of breaking required ligatures was observed with SansBullshitSans which sets requiredFeatureIndex to reference the feature tagged 'liga' (it could also have done this by using 'rlig'). While this is obviously a novelty font, it would seem quite useful to support required ligatures by not breaking them.