When you increase the window size of a JavaFX application by dragging the window border with the mouse you see artifacts of the black window background. This makes JavaFX look slow and ugly. The size of the artifacts depends on the drag speed. I wonder why you can see these background artifacts if a not too complex scene graph can be rendered at 60 Hz (the screen refresh rate). If window and scene drawing were properly synchronized such artifacts should never be visible as long as the scene can be rendered fast enough.

If this problem cannot be solved at its root it might help to at least change the window background color to the background color of the scene, so that these artifacts become less obvious and disturbing.