This question gets asked a lot; try searching the forums for pixel-perfect rendering for a full answer. But the short answer is add this to your Config.prc file:
textures-power-2 up
Reparent your text to render2d instead of aspect2d. Or use relative setPos to render2d.
If you are holding the mouse button when you move the mouse pointer out of the window, the window will continue to track the mouse movement. If you are not holding the mouse button, the window will stop tracking the mouse movement when it goes beyond the window. Is this the behavior you observe? What behavior would you expect instead?
Please, give a short explanation of trick with NodePath(yourOnscreenText).setPos(render2d,x,y,z), why render2d instade aspect2d ? How is it OnscreenText in aspect2d, but his coordinates in render2d ?