So even now that I know that the $extralegal moves are used for highlighting, by studying the JavaScript, I still have the dilemma of whether to push a move a-b simply as a-b, or as a-b; pass. I now solve that by letting the JavaScript highlight the destination of the first leg also as possible destination of the second leg to select a move without side effect, and have it add the pass to the move just before submitting it, when that move gets selected by a third click.
This is a kludge that I wish to avoid. If the legalList includes a move by itself, and it includes that same move as the first part of multi-part moves, but none of these have pass as the second part, there are three possible interpretations of this, and it does not know how to rule between them. One interpretation is that an incomplete move is being allowed, and a continuation will be required on the next step. The second interpretation is that it is a complete legal move by itself. The third interpretation is a mishmash of the first two. It is that it is an incomplete move as written, but adding pass as the second part will make it a complete legal move. For the first two interpretations, it would not be appropriate to add pass. It is only for the third, which happens to be the case for games your applet has programmed, where it would be appropriate to add pass.
This is a kludge that I wish to avoid. If the legalList includes a move by itself, and it includes that same move as the first part of multi-part moves, but none of these have pass as the second part, there are three possible interpretations of this, and it does not know how to rule between them. One interpretation is that an incomplete move is being allowed, and a continuation will be required on the next step. The second interpretation is that it is a complete legal move by itself. The third interpretation is a mishmash of the first two. It is that it is an incomplete move as written, but adding pass as the second part will make it a complete legal move. For the first two interpretations, it would not be appropriate to add pass. It is only for the third, which happens to be the case for games your applet has programmed, where it would be appropriate to add pass.