One reason is that it makes it more consistent with other backends, in that closing them is irreversible because it destroys the `OpenGL` context. Not destroying the window can leave lingering OpenGL contexts that screw up other windows, so if more than one window is open things get messy. (We discovered this in testing.) My vote is to leave it this way until we hit a use case where we need the re-open capability, but if we want to build it in now, we could probably add something like `destroy_on_close=True` default `kwarg` to the `Canvas` constructor, that could be `False` only if `qt` is the backend...?