Does U+034F COMBINING GRAPHEME JOINER affect display of combining character sequences?
A. No. It does not impact cursive joining or ligation (contrast U+200C ZERO WIDTH NON-JOINER and U+200D ZERO WIDTH JOINER). And the CGJ does not have any visible display of its own. Of course, as for any such character in the Unicode Standard with no visible display, it is always possible to use a visible glyph when deliberately showing hidden characters, as for an editor’s Show Symbol or Show Hidden mode.