Simple GWT 'sticky' scroll handler - like the one used in http://boardcast.it - it will cause scrolling to 'stick' from headerTopTriggerInPixels to headerBottomTriggerInPixels with a gravity towards initialScrollPosition. Useful if you have content you wish to 'hide' above the normal viewable area.

StickyTopScrollHandler.java

  import com.google.gwt.user.client.Window;
  
  public class StickyTopScrollHandler implements Window.ScrollHandler {

        private final int   headerTopTriggerInPixels;
        private final int   headerBottomTriggerInPixels;
        private final int   initialScrollPosition;
        private Timer timer;
        private int oldTop;

        public StickyTopScrollHandler(int headerTopTriggerInPixels, int headerBottomTriggerInPixels, int initialScrollPosition) {
            this.headerTopTriggerInPixels = headerTopTriggerInPixels;
            this.headerBottomTriggerInPixels = headerBottomTriggerInPixels;
            this.initialScrollPosition = initialScrollPosition;
        }

        @Override public void onWindowScroll(Window.ScrollEvent event) {
            
            if (Window.getScrollTop() > headerTopTriggerInPixels
                && Window.getScrollTop() < headerBottomTriggerInPixels
                && timer == null) {
                oldTop = Window.getScrollTop();
                timer = new Timer() {
                    int count = headerBottomTriggerInPixels - headerTopTriggerInPixels;

                    @Override public void run() {
                        count--;
                        if (Window.getScrollTop() == initialScrollPosition
                            || count == 0
                            || Window.getScrollTop() < headerTopTriggerInPixels
                            || Window.getScrollTop() > headerBottomTriggerInPixels) {
                            timer.cancel();
                            timer = null;
                        }
                        final int delta = (int) Math.signum(initialScrollPosition - Window.getScrollTop());
                        if (oldTop == Window.getScrollTop() || ((int) Math.signum(Window.getScrollTop() - oldTop)) != delta) {
                            Window.scrollTo(Window.getScrollLeft(), Window.getScrollTop() + delta);
                        }
                    }
                };
                timer.scheduleRepeating(10);
            }
        }

    }