In the font for this test case, the glyphs Agrave and Udieresis pass five arguments to the endchar command. For a description of this particular variant of the endchar operator, see Adobe PostScript Technical Note #5177, Appendix C, at the bottom of page 35. See also OpenType.js bug 117 and fontkit bug 322.

If the implementation under test works correctly, you should see the glyphs À and Ü.