invalid cow squares remain highlighted

[evemartin]

Describe the bug
Sometimes after a cow is placed on an invalid square, the square remains highlighted when the user moves their mouse away.

To Reproduce
Steps to reproduce the behaviour:

1. Go to Level Editor
2. Add a cow to the grid and drag it to be right on the leftmost edge of an invalid square (see screenshot)
3. Drop the cow in that position and move your mouse away to the left, i.e. do not move your mouse over the square
4. Notice that the square stays highlighted white when it should not be highlighted at all
5. Bonus: when you move your mouse over and off the highlighted square, only then does it unhighlight

Expected behaviour
The cow square should unhighlight as soon as the cow is dropped

Screenshots