Why can a control draw outside its bounding box?

Question

Here's an example image of what I mean: example

The gray rectangle is the bounding box of a control that draws the blue lines and dots in it's OnRender(...) method. The red ovals mark places where it happens.

• Why is that possible?
• How can it be avoided?

Answer 1

Here's the perfect answer to my second question, at least when using a rectangular shaped control:

<object ClipToBounds="True" />

More details on the MSDN.