Account for the tabs height when they are at the bottom.

[avautour]

When the tabs are at the bottom, tabs were being clipped by the contents when the window size approached its minimum height.

To reproduce:

1. Start the AutoHideDragNDropExample example application
2. Dock the Properties at the bottom
3. Move Tab1 to be tabbed with the properties
4. Resize the tabbed container to its minimum height.