Funny enough this disappearance of pieces had nothing to do with the specific implementation of Borderline, but was due to an existing bug in the Diagram script for a feature that had never been really been used. In particular, there was code to suppress moves that blow up / capture your own royal, and this was tested when the piece doing that was temporarily removed from the board. The code forgot to put it back. And it was also applying this code to non-royals that were counting towards the baring rule (which is the rule Borderline uses, as the normal royalty mechanism doesn't work with a shared King), which was also not intended. A piece that could capture the King (which at that time would be considered an own piece, as you should also be able to move it) during the look-ahead would thus disappear.
This should be fixed now (after you flush the browser cache).
Funny enough this disappearance of pieces had nothing to do with the specific implementation of Borderline, but was due to an existing bug in the Diagram script for a feature that had never been really been used. In particular, there was code to suppress moves that blow up / capture your own royal, and this was tested when the piece doing that was temporarily removed from the board. The code forgot to put it back. And it was also applying this code to non-royals that were counting towards the baring rule (which is the rule Borderline uses, as the normal royalty mechanism doesn't work with a shared King), which was also not intended. A piece that could capture the King (which at that time would be considered an own piece, as you should also be able to move it) during the look-ahead would thus disappear.
This should be fixed now (after you flush the browser cache).