The character joiner api is invoked at render time. However, grapheme clusters need to be known no later than line breaking/overflow time, including reflow on window size resize. The logical place to detect clusters would seem to in the InputHandler print function. This can be combined with wide-character detection, using a single lookup. DomTerm uses an efficient combined trie lookup. which I have ported/converted to TypeScript (not yet checked in).

As a "heads up": I'm working on a solution for this. Unfortunately, this involves a re-design of the BufferLine implementation. On the other hand, the re-write has other benefits, which I will discuss later.