A sliding door or window assembly comprising: a. a frame having a bottom sill, b. a first fixed panel and a second fixed panel, said first and second fixed panels being arranged in the frame at opposite sides of the frame and such that an outer surface of the first fixed panel is essentially co-planar with an outer surface of the second fixed panel, c. a first sliding panel and a second sliding panel positioned between the first and second fixed panels (3,4), said first and second sliding panels (5,6) each being displaceable between a closed position and an open position along a vector which has a major component along a direction which is parallel to the plane of the fixed panels, said first and second sliding panel (5,6) arranged such that outer surfaces of said first and second sliding panels are arranged essentially co-planar with the outer surfaces of the first and the second fixed panel (3,4) when the first and second sliding panels (5,6) are in their closed positions, d. a vertically arranged mullion bar arranged between the first and second sliding panels and arranged to abut both the first and the second sliding panels in a sealed manner when the first and second sliding panels are in their closed positions, and being connected to the second sliding panel such that when the second sliding panel displaces to its open position, the mullion bar follows the second sliding panel, e. a first and a second track arranged along a direction which is essentially parallel with the plane of the first and second fixed panels respectively, f. a first and a second wagon arranged to be displaceable along the first and second tracks respectively, g. a first displacement mechanism connecting the first wagon and the first sliding panel and a second displacement mechanism connecting the second wagon and the second sliding panel, h. the first and second displacement mechanisms being arranged such that the first and second sliding panels (5,6) are displaceable with respect to the first and second wagons respectively in a direction having a vector component which is perpendicular to the plane of the sliding panels, i. characterized in that said assembly further comprises a mullion bar displacement mechanism (52,56,62,76) connecting the mullion bar and the second sliding panel, said mullion bar displacement mechanism (52,56,62,76) being arranged such that at least a portion of the mullion bar is displaceable with respect to the second sliding panel. a. a frame having a bottom sill, b. a first fixed panel and a second fixed panel, said first and second fixed panels being arranged in the frame at opposite sides of the frame and such that an outer surface of the first fixed panel is essentially co-planar with an outer surface of the second fixed panel, c. a first sliding panel and a second sliding panel positioned between the first and second fixed panels (3,4), said first and second sliding panels (5,6) each being displaceable between a closed position and an open position along a vector which has a major component along a direction which is parallel to the plane of the fixed panels, said first and second sliding panel (5,6) arranged such that outer surfaces of said first and second sliding panels are arranged essentially co-planar with the outer surfaces of the first and the second fixed panel (3,4) when the first and second sliding panels (5,6) are in their closed positions, d. a vertically arranged mullion bar arranged between the first and second sliding panels and arranged to abut both the first and the second sliding panels in a sealed manner when the first and second sliding panels are in their closed positions, and being connected to the second sliding panel such that when the second sliding panel displaces to its open position, the mullion bar follows the second sliding panel, e. a first and a second track arranged along a direction which is essentially parallel with the plane of the first and second fixed panels respectively, f. a first and a second wagon arranged to be displaceable along the first and second tracks respectively, g. a first displacement mechanism connecting the first wagon and the first sliding panel and a second displacement mechanism connecting the second wagon and the second sliding panel, h. the first and second displacement mechanisms being arranged such that the first and second sliding panels (5,6) are displaceable with respect to the first and second wagons respectively in a direction having a vector component which is perpendicular to the plane of the sliding panels, i. characterized in that said assembly further comprises a mullion bar displacement mechanism (52,56,62,76) connecting the mullion bar and the second sliding panel, said mullion bar displacement mechanism (52,56,62,76) being arranged such that at least a portion of the mullion bar is displaceable with respect to the second sliding panel.