This is about dragging a tab out of or into a browser window, and letting the compositor know about it, so it can move and place the window accordingly. Apps don't get to place windows themselves.
I used to do apps with QT (as well as with Java) and when creating a window, I only needed to say, "new window of that preferred size please", then the engine would make the window of that size if possible.
Now, maybe QT did things more in depth behind the scene, I don't know.
In those cases, I agree.
But for a tiling window manager like w3m, I don't see the application having a say in position and location.
Hence I didn't think that the app has so much to do with creating windows. Just my thought.