Tk_GetVRootGeometry(tkwin, xPtr, yPtr, widthPtr, heightPtr)
Tk_GetVRootGeometry returns geometry information about the virtual root window associated with tkwin. The “associated” virtual root is the one in which tkwin's nearest top-level ancestor (or tkwin itself if it is a top-level window) has been reparented by the window manager. This window is identified by a __SWM_ROOT or __WM_ROOT property placed on the top-level window by the window manager. If tkwin is not associated with a virtual root (e.g. because the window manager does not use virtual roots) then *xPtr and *yPtr will be set to 0 and *widthPtr and *heightPtr will be set to the dimensions of the screen containing tkwin.