pane-screen-internal-geometry returns the internal geometry of the "monitor" in which the interface that contains
is displayed. A "monitor" is typically a physical monitor, but can be anything that the underlying GUI system considers a monitor.
must be inside an interface that is already displayed.
pane-screen-internal-geometry returns the internal geometry of the monitor on which this interface is displayed. If the interface spreads across multiple monitors, it returns the geometry for the monitor on which the largest area of the interface is displayed.
The internal geometry of a monitor is a rectangle which excludes "system areas" like taskbars and global menu bars and so on. Examples of these include the Windows taskbar, the Mac OS X menu bar, and the Mac OS X Dock. See screen-internal-geometry for information about displaying CAPI windows in system areas.
On GTK+ the internal geometry is of the workspace in which the interface is displayed. When there are multiple monitors these values may be incorrect. You can check the number of monitors by screen-monitor-geometries.
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017