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...

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Original road vehicle list window with new shade button.

In the above image the road vehicle list is untouched, the only difference being the new shade windows button.

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New window, much smaller since it is shaded.

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.

Development

Shade Windows was based off of the patch by erikjanp.