If you click and hold on the sliderbar, moving the mouse-up moves the page
up and down.  Good.  That's what it is supposed to do.

Now, if you clock and hold on the sliderbar, and release the mouse button
after traveling into the grey-space above the top of the scroll bar and
release there, the release seems to get lost and moving the mouse up and
down anywhere causes the same scroll-bar activity to occur.

So the steps to repeat this bug:

0) make sure you have a page which needs the sliderbar to see the whole
thing and move the slider down a ways and release the sliderbar
1) left-click-and-hold on the scrollbar sliderbar
2) Move the sliderbar all the way up until it hits the decoration at the
top of the scrollbar and continue holding the mouse button down
3) continue moving the mouse upwards.  since the sliderbar can't track the
mouse pointer any more, you can keep moving the mouse-upwards past the
decoration at the top of the scrollbar and into the bookmark menu area
4) now release the mouse
5) move the mouse downwards back past the original point at which you
grabbed the scrollbar, you will see the screen scrolling as though you
still had the left button depressed

Note: if you start with the slider all of the way at the top of the
scrollbar (ie, no screen motion occurs while the mouse button is held down)
then the bug doesn't appear