Tk_Window Tk_CoordsToWindow(rootX, rootY, tkwin)
Tk_CoordsToWindow locates the window that contains a given point. The point is specified in root coordinates with rootX and rootY (if a virtual-root window manager is in use then rootX and rootY are in the coordinate system of the virtual root window). The return value from the procedure is a token for the window that contains the given point. If the point is not in any window, or if the containing window is not in the same application as tkwin, then NULL is returned.
The containing window is decided using the same rules that determine which window contains the mouse cursor: if a parent and a child both contain the point then the child gets preference, and if two siblings both contain the point then the highest one in the stacking order (i.e. the one that's visible on the screen) gets preference.