dragging works

1 files changed, 5 insertions, 2 deletions

diff --git a/grid.py b/grid.py
index bcb0a49..624a631 100644
--- a/grid.py
+++ b/grid.py
@@ -91,8 +91,11 @@ class Grid:
 
     def xy_to_grid(self, x, y):
         ''' Convert from sprite x,y to grid index. '''
-        return COL * int((y - self.top) / self.yinc) + \
-               int((x - self.left) / self.xinc)
+        if x > self.left:
+            return COL * int((y - self.top) / self.yinc) + \
+                   int((x - self.left) / self.xinc)
+        else:
+            return None
 
     def grid_to_xy(self, i):
         ''' Convert from grid index to sprite x,y. '''