To reproduce:

1. Start a new game.
2. Click the Custom difficulty.
3. Focus the Height input box and try to enter the value 33 from the keyboard.

Expected behavior: the Height input box value is set to 33.

Observed behavior: a 30×16 game starts immediately when the digit 3 is pressed.

I strongly suspect these lines of code (gnome-mines.vala:275–278 in current master):

        set_accels_for_action ("app.small-size", {"1"});
        set_accels_for_action ("app.medium-size", {"2"});
        set_accels_for_action ("app.large-size", {"3"});
        set_accels_for_action ("app.custom-size", {"4"});