All of of this can be seen in your demo, so I don't need to send code.
lets assume the window is 200 wide.
1) Assume that the window that is to be slide out is not visible
2) I then hit the slide down button.
3) The first thing I should see is a horizontal line that is 200 wide. This horizontal line should remain at the origin of slide out until the entire window is completely out.
This is where my analogy may be weird. Cut a horizontal slit in your canvas that is 200 wide and slide the window out. Upon the window being fully exposed, heal the slit.
In the real world things just dont slide out from nothingness, they always have an origin. In your implementation, it is nothingness.