Shade Windows is a new feature in trunk since r18588. It allows you to shade windows (as the name says) so that they don't take up as much space as they normally do. See the following pictures...
In the above image the road vehicle list is untouched, the only difference being the new shade windows button.
To switch between both forms, either click at the shade box (the grey rectangle with the arrow up or down) in the title bar, or use the scroll wheel at the caption or the shade box.
Shade Windows was based off of the patch by erikjanp.