When scrolling by dragging the scrollbar thumb; if mouse button is released
over the menubar or the toolbars, the release event is not propagated to
the scrollbar widget, so the dragging is not terminated. Thus, moving the
mouse pointer back into the rendering area (without any buttons pressed),
causes scrolling to proceed as if the button is still pressed. Button
releases that are made elsewhere (including over the statusbar and outside
the window) seem to be handled appropriately.

This misbehavior has been observed for as long as I have been using Galeon.
It applies to the vertical scrollbar as well as the horizontal scrollbar
when present.