Any of the return location arguments to this function may be NULL, if you aren’t interested in getting the value of that field.
The X and Y coordinates returned are relative to the parent window of window, which for toplevels usually means relative to the window decorations (titlebar, etc.) rather than relative to the root window (screen-size background window).
On the X11 platform, the geometry is obtained from the X server, so reflects the latest position of window; this may be out-of-sync with the position of window delivered in the most-recently-processed GdkEventConfigure Window.getPosition in contrast gets the position from the most recent configure event.
Note: If window is not a toplevel, it is much better to call Window.getPosition, Window.getWidth and Window.getHeight instead, because it avoids the roundtrip to the X server and because these functions support the full 32-bit coordinate space, whereas Window.getGeometry is restricted to the 16-bit coordinates of X11.