JViewport programatically scrolling

I created my own scroll pane by extending JViewport.  This is a chat window, so it's size is dynamic.  Everything works great except it won't stay scrolled to the bottom, it is always on the very top.  When someone types I want it to scroll so you can see what was typed.  I have a  JTextPane within the viewport.  I use the setCaret method to position the caret to the bottom the the textpane.  I also do this.

 public void autoScroll(){

        if(this.getView().getHeight()>this.getHeight()){
            int max = getMaxYExtent();
            Point mpt = new Point(pt.x, max);
            this.setViewPosition(mpt);
        }

 private int getMaxYExtent(){
       
        return this.getView().getSize().height-this.getSize().height;
    }

This does work.  I can see it scroll but something resets it back.  I tried debuggin and checking what happened after this and it just goes to a bunch of my paint methods.  What resets the viewport to the top?