Scrolling using scrollbar -- click left mouse button on it, hold the
button, move the pointer from the scrollbar e.g. on galeon's toolbar (or
statusbar, or another solely galeon's part of interface) (still holding the
LMB), then release the button there. If you then move the pointer to the
main viewing window, the scrollbar is following it (as if you were still
holding the LMB), until you click LMB.

The problem (probably) originates from Mozilla (gecko engine) (which is
responsible for the scrollbar, as far as i can tell) not receiving the
release notify event (if that is generated above the Galeon (gtk) owned
windows).