A screen assembly that comprises: a support structure; a roller assembly mounted on the support structure; a flexible panel with a distal end mounted on the roller assembly so that the flexible panel can be wound onto and off the roller assembly as the flexible panel is retracted and extended; two opposed track assemblies (20, 22) arranged on the support structure, sides of the flexible panel and each track assembly (20, 22) being configured so that the sides of the flexible panel can slide to and fro, longitudinally, within the track assemblies (20, 22) as the flexible panel is retracted and extended, at least one of the track assemblies (20, 22) having a static support that is fixed relative to the support structure and a dynamic support that is displaceable relative to the static support, a bead guide at or near an end of at least one of the dynamic support and the static support, proximate the roller assembly; an adjustment mechanism that is operatively connected to the static and dynamic supports (24, 26) to adjust a position of the dynamic support relative to the static support; an elongate guide assembly that is mounted on a proximal end of the flexible panel to guide movement of the flexible panel with respect to the track assemblies (20, 22); an insert mounted on at least one end of the elongate guide assembly; and the dynamic support of said at least one of the track assemblies (20, 22) including a runner in which the insert is received to slide or roll with respect to the dynamic support as the flexible panel is retracted and extended; in which the support structure includes a top side member and an opposed bottom side member, the roller assembly being mounted between the side members; such that the support structure and the roller assembly are configured for generally vertical orientation so that the flexible panel can extend generally horizontally, the bead guide being positioned to support a bead of the flexible panel during such horizontal extension; characterized in that said adjustment mechanism is interposed between the static and dynamic supports (24, 26) and is operable to adjust the static and dynamic supports (24, 26) relative to each other along a line perpendicular to a longitudinal axis of the static support. a support structure; a roller assembly mounted on the support structure; a flexible panel with a distal end mounted on the roller assembly so that the flexible panel can be wound onto and off the roller assembly as the flexible panel is retracted and extended; two opposed track assemblies (20, 22) arranged on the support structure, sides of the flexible panel and each track assembly (20, 22) being configured so that the sides of the flexible panel can slide to and fro, longitudinally, within the track assemblies (20, 22) as the flexible panel is retracted and extended, at least one of the track assemblies (20, 22) having a static support that is fixed relative to the support structure and a dynamic support that is displaceable relative to the static support, a bead guide at or near an end of at least one of the dynamic support and the static support, proximate the roller assembly; an adjustment mechanism that is operatively connected to the static and dynamic supports (24, 26) to adjust a position of the dynamic support relative to the static support; an elongate guide assembly that is mounted on a proximal end of the flexible panel to guide movement of the flexible panel with respect to the track assemblies (20, 22); an insert mounted on at least one end of the elongate guide assembly; and the dynamic support of said at least one of the track assemblies (20, 22) including a runner in which the insert is received to slide or roll with respect to the dynamic support as the flexible panel is retracted and extended; in which the support structure includes a top side member and an opposed bottom side member, the roller assembly being mounted between the side members; such that the support structure and the roller assembly are configured for generally vertical orientation so that the flexible panel can extend generally horizontally, the bead guide being positioned to support a bead of the flexible panel during such horizontal extension; characterized in that said adjustment mechanism is interposed between the static and dynamic supports (24, 26) and is operable to adjust the static and dynamic supports (24, 26) relative to each other along a line perpendicular to a longitudinal axis of the static support.