I also found another bug where a piece that promotes to a burner doesn't show in the promotion options and then promotes to a white burner if the moving piece was black. Take for example the Chess Knight in the diagram in my last comment.
[Edit]: Pieces that promote to burners upon being asked for promotion choice (at least for Shogi-style promotion) will also burn pieces regardless of whether promotion is accepted or deferred, even if the original piece does not burn.
I finally understand what is the problem here. There actually was an avelanche of bugs, some stupid (like accidentally masking off the color bit, or forgetting to mask the 'also burns' bit from the code of the promoted piece), and some more fundamental (introduced by the legal highlighting). The stupid ones are fixed now, so that black promotes to black pieces, and the promoted piece does show up on the purple squares.
The problem with the spurious burning is more fundamental, though: the NewClick input routine combined all moves with the same step to a single one for highlighting, making a list of all posible promotion choices for that move. This was fine when burning was only indicated by a different code for the promotion pieces, as it is when the pseudo-legal moves are first generated. Before the moves can be used they are 'scored', however, which also includes identifying which pieces get burned, and writing those into the move as extra locust squares. This used to be done after the user had selected the move and promotion pieces.
But for the legal highlighting it has to be done for all the moves of the selected piece, to make those in order to test legality even before the user selects one. But this interferes with combining the promotion choices when one of those would burn, and the other not. I am not sure yet how to solve this. It is conceivable that one of the promotion choices is legal, and the other not. (E.g. because you burn a piece blocking a slider attack on the King, or fail to burn a hopper mount.)
I finally understand what is the problem here. There actually was an avelanche of bugs, some stupid (like accidentally masking off the color bit, or forgetting to mask the 'also burns' bit from the code of the promoted piece), and some more fundamental (introduced by the legal highlighting). The stupid ones are fixed now, so that black promotes to black pieces, and the promoted piece does show up on the purple squares.
The problem with the spurious burning is more fundamental, though: the NewClick input routine combined all moves with the same step to a single one for highlighting, making a list of all posible promotion choices for that move. This was fine when burning was only indicated by a different code for the promotion pieces, as it is when the pseudo-legal moves are first generated. Before the moves can be used they are 'scored', however, which also includes identifying which pieces get burned, and writing those into the move as extra locust squares. This used to be done after the user had selected the move and promotion pieces.
But for the legal highlighting it has to be done for all the moves of the selected piece, to make those in order to test legality even before the user selects one. But this interferes with combining the promotion choices when one of those would burn, and the other not. I am not sure yet how to solve this. It is conceivable that one of the promotion choices is legal, and the other not. (E.g. because you burn a piece blocking a slider attack on the King, or fail to burn a hopper mount.)