Disable a UI element when a mouse click occurs outside its bounding rectangle.

UIExtensions.cs

public static class UIExtensions
{
    public static bool HideIfClickedOutside(this GameObject panel) {
        if (Input.GetMouseButton(0) && panel.activeSelf && 
            !RectTransformUtility.RectangleContainsScreenPoint(
                panel.GetComponent<RectTransform>(), 
                Input.mousePosition, 
                null)) {
            panel.SetActive(false);
            return false;
        }

        return true;
    }
}