A flush-flush horizontal vehicle window assembly is provided that includes a sliding panel with guide pins, formed with a load-bearing shoulder, on sliding panel corners, and a frame defining a window opening. The frame is attached to two fixed panels. Upper/lower tracks are attached above/below the window opening. Each track has a leading and a trailing guide path formed through a surface of the respective tracks, where the pins partially project into the guide paths and where load-bearing shoulder surfaces smoothly and reliably slide on the track surfaces. Each leading guide path is “S” shaped and each trailing guide path is shaped in a perpendicular manner at one end, which causes the sliding panel to smoothly enter and seal the window opening in a flush manner. An aesthetically pleasing flush-flush slider results when the sliding panel is in the plane of the window opening and further in the plane of the outer vehicle body panel.