Mike Dunn was curious about the intended purpose of HDS_FILTERBAR.

The HDS_FILTERBAR style adds a row below the header control consisting of an edit control and a funnel icon. The funnel icon presumably represents a coffee filter, because after all, everybody in the world drinks coffee as much as people in Seattle. (Developers think they're so clever.)

Mike points out that new features of the common controls were nearly always used by whatever version of Windows or Internet Explorer shipped that new version. The HDS_FILTERBAR style is a notable exception. What happened?

I believe the HDS_FILTERBAR feature was originally intended for use by Active Directory; my guess is that dialogs like Find Computer would have taken advantage of it. For whatever reason, that feature was cut from Active Directory, which is why you didn't see anybody using it. However, the feature was cut after the code for the feature was already written and checked into the common controls under the style HDS_FILTERBAR.

The Active Directory team either forgot to tell the Common Controls team, "Hey, you know that feature we asked you to write for us? Yeah, we don't need it after all," or they did, and the Common Controls team said, "Well, we already wrote it, and we don't want to take the risk that removing it won't introduce a bug, so we'll just leave it in. Maybe somebody else can find a use for it."

The result was a feature in the header control that nobody used. And since nobody used it, I wouldn't be surprised if it's a little buggy. (We already know that it's more than little ugly.)