Transforms window coordinates from a parent window to a child window, where the parent window is the normal parent as returned by Window.getParent for normal windows, and the window's embedder as returned by OffscreenWindow.getEmbedder for offscreen windows.
For normal windows, calling this function is equivalent to subtracting the return values of Window.getPosition from the parent coordinates. For offscreen windows however (which can be arbitrarily transformed), this function calls the GdkWindow::from-embedder: signal to translate the coordinates.
You should always use this function when writing generic code that walks down a window hierarchy.
See also: Window.coordsToParent