Scid's GUI has two modes. In Docked Windows mode (the default), most windows are tiled - or docked - into a single large window. Docking is enabled or disabled in Options-> Windows, where you can also load any of the three built-in layouts, or save your own Window arrangements. The current window arrangement can also be saved via Options-> Save Layout.
Arranging Docked Windows is a little difficult. One can right-click any tab (except the main board) and select to move to the side of the current pane, or one can drag the tab and group it with other tabs in another paned window. Windows can also be undocked by right-clicking the tab, though Mac OS X does not support undocking windows.
Window focus automatically follows the mouse around, and also impacts which keyboard bindings are active. Most bindings are active when the mouse is over the Main Board.
The window's menus are accessible via clicking the arrow in the window tab.
With the focus on a window tab, one can press the Tab key to move between window panes. Control+Tab switches between active tabs.
If Scid fails to start, use the -nodock command-line option to start in undocked mode.