That happens in TeX as well, the width of “efficient” does not necessarily equal to the width of “ef” + “ficient” due to the presence of ligatures, and TeX deal

That happens in TeX as well, the width of “efficient” does not necessarily equal to the width of “ef” + “ficient” due to the presence of ligatures, and TeX deals with it by storing the width of the full ligature as well as the pre and post-line break widths (for all possible line breaks in the middle of paragraphs) and takes them into consideration when calculating line breaks. This can be generalized to any situation where line breaking changes the width of a chunk of the text.