In Windows, the desktop space is the area inside the MDI window (sometimes referred to as the client area). This doesn’t include any virtual space available through the scrolling of the MDI window.
In the Mac OS, the desktop space is the area on the main monitor excluding the menu bars. The main monitor is where the menu bar is located.
Returns 956 in Windows when there is a single monitor and its MDI is set to 1280 x 1024.
Returns 1178 in the Mac OS when there is a single monitor and its resolution is set to 1900 x 1200.