[fowlplay4]

1. onMouseDown: Determine the tile the mouse is currently over and use it as your top-left reference point.
2. onMouseDragged: Determine the tile the mouse is currently over and use it as your bottom-right reference point.
3. onMouseUp: Do whatever with your selection.

I believe:

tile_x = mouse_x - (mouse_x % tile_width);
tile_y = mouse_y - (mouse_y % tile_height);

Will get you the top-left x and y of the tile your mouse is currently over.

% is for modulus which basically gives you the remainder from dividing a number. I.e: 20 % 16 equals 4.