What's the name of the behaviour of the "buddy list" window of pidgin?

I noticed the following feature of the "buddy list" window of pidgin on windows: If you drag the window not far from the top right corner of the desktop, the following things happen:

• The appearance of the border slightly changes:

Before:

After:

• It docks on the right of the desktop
• And here is the most interesting: it reserves the space on the desktop. All other maximized windows are reduced by the width of pigin window. All other windows behave as if the width of the desktop was shorter by the width of the pidgin window

Q: What is the name of the feature implemented by the last point ?

You create custom toolbars that the desktop observes when it calculates the working area with the SHAppBarMessage() api function. Check the MSDN Library page for documentation, it also googles really well to find sample code.