VisIt uses different window layouts to organize vis windows so they all fit on the screen. Changing the window layout typically resizes all of the vis windows and moves them into a tiled formation. If there are not enough vis windows to complete the desired layout, VisIt creates new vis windows until the layout is complete. You can change the layout selecting a new layout from the Layouts menu located in the Main Window's Window menu or you can click on a layout icon in the vis window's Toolbar.