Display of Grapheme Clusters. Grapheme clusters are not the same as ligatures. For example, the grapheme cluster “ch” in Slovak is not normally a ligature and, conversely, the ligature “fi” is not a grapheme cluster. Default grapheme clusters do not necessarily reflect text display. For example, the sequence <f, i> may be displayed as a single glyph on the screen, but would still be two grapheme clusters.
Right, so "f" and "i" are two separate grapheme clusters (and always are), but might map to one rendered glyph under certain fonts that combine them into a ligature. Grapheme clusters have a specific definition that doesn't depend on the font in use. That definition is updated over time as the Unicode standard is updated, which means it can vary over time, but it does not change based on the font.
