Jonas Koelker points out that the backspace key didn't work in GTK

Guess, because Guess expected ^H whereas GTK generated ^?. Other
puzzles that use Backspace do it by being prepared to see either,
which seems wasteful. Now the midend normalises both into ^H, so
front ends can generate whichever they like while puzzles can
safely just look for ^H.

[originally from svn r8786]

filling.c

@@ -1097,7 +1097,6 @@ static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
       case '\r':
       case '\n':
       case '\b':
-      case '\177':
         button = 0;
         break;
       default: