This method scrolls into view the thumbnails contained in the line previous to the band in current view. Note: For thumbnails arranged in a single horizontal line, the scroll position will shift to a single thumbnail previous to the current scroll position. For thumbnails layout in a grid format, this control always has scroll bar on the right, therefore, the scroll position will move to the previous band of thumbnails. When the thumbnail layout has a single vertical column (a special case of grid with one column), the scroll position moves into view one thumbnail above the current position.

Reference

ImGearThumbnailList class
ImGearThumbnailList Members