The movie above is a simple zooming animation. A movieclip contains a 30x30 rectangular shape with stroke set as 3-pixel-wide. Each frame, the _xscale increase 50 and _yscale increase 10. After 20 frames, I zoomed it back.
It is really simple, right ?
No ! I add some trick in that movie.
You can try to do this simple animation and see what you get. Possibly, you will get a movie shown below.
When the large rectangle is zooming out, residual strokes appear above and below. Not very clean. This is documented issue of Flash player. Here is just a demonstration.
To get rid of this annoying remnants, there are many ways. One of them is "force back ground to update the painting". Here I add some off-stage objects and make _root._visible=false; _root._visible=true; That force player to refresh the back ground bounded by those two off screen objects. (Becareful, some old version player might not repaint the off screen objects.)
Also note that, refresh _root is a hard job for player. It add CPU load.