This can be disabled by running through the frames first without painting. In most cases saving animation in Internet Explorer fails, and may even crash the browser. Test the save function before spending too much time. Points moved off the screen are erased. This usually renders unexpected results.
Layers are independent screens or overlays.
Drawing on one layer will not effect the drawing on another. Using layers in combination with other features should help you build an animation quickly. When the animation is played the higher numbered layer is drawn on top. All three layers are visible but you are only drawing to the layer you have selected. Avoid duplicating animation longer then 100 frames.