user/src/com/google/gwt/user/client/ui/NativeHorizontalScrollbarTransparent.gss

@def OPACITY_DURATION 350ms;

.nativeHorizontalScrollbar {
  opacity: 0.7;
  filter: alpha(opacity = 70);
  -webkit-transition: opacity OPACITY_DURATION;
  -moz-transition: opacity OPACITY_DURATION;
  -o-transition: opacity OPACITY_DURATION;
  transition: opacity OPACITY_DURATION;
}

.nativeHorizontalScrollbar:hover {
  opacity: 1;
  filter: alpha(opacity = 100);
}