If you create a UI element underneath the mouse, the element never gets the mouse over event. When an element gets created, there should be something that checks the mouse position and pro-actively triggers the mouse move event on that widget so the user does not have to move the mouse a pixel in a direction to allow clicking. This can be easily tested by creating a button underneath the mouse on right-click, and observe the button is unresponsive to clicks and is not highlighted until you move your mouse a pixel or two. This affects all controls, not just buttons.
If you create a UI element underneath the mouse, the element never gets the mouse over event. When an element gets created, there should be something that checks the mouse position and pro-actively triggers the mouse move event on that widget so the user does not have to move the mouse a pixel in a direction to allow clicking. This can be easily tested by creating a button underneath the mouse on right-click, and observe the button is unresponsive to clicks and is not highlighted until you move your mouse a pixel or two. This affects all controls, not just buttons.