Fixed edgecase when mouse is held-down outside of window.

conway.py

@@ -108,8 +108,9 @@ class Board:
     def fill_crowding(self):
         for row in self.matrix:
             for cell in row:
-                crowding_number = self.calculate_crowding_number(cell.position, XYPair(1,1))
-                cell.set_crowding_number(crowding_number)
+                cell.set_crowding_number(
+                    self.calculate_crowding_number(cell.position, XYPair(1,1))
+                )
     
     def evolve_state(self):
         #calculate crowding numbers before updating any state, otherwise you'll be unhappy
@@ -143,7 +144,10 @@ class Board:
         x = pixel_to_index(mouse_pos[0], dimensions.x, render_info.padding.x)
         y = pixel_to_index(mouse_pos[1], dimensions.y, render_info.padding.y)
 
+        try:
             cell = self.matrix[y][x]
+        except IndexError:
+            return #this exception corresponds to the case of the mouse being held down outside the game window
 
         if not self.click_state:
             if cell.state == CellStates.ALIVE:
@@ -189,7 +193,7 @@ def main():
 
     while running:
 
-        dt = clock.tick(60)
+        dt = clock.tick(144)
         board.draw(screen)
         pygame.display.flip()