Fix ListView scrollbar dragging

With af08a56 properly passing down relativized events we don't have to implement
to scrollbar dragging logic ourselves anymore, but can rely on the one in
cursive.

This also fixes #839

src/ui/listview.rs

@@ -305,8 +305,6 @@ impl<I: ListItem> View for ListView<I> {
                 offset,
             } => {
                 if self.scroller.get_show_scrollbars()
-                    && position.y > 0
-                    && position.y <= self.last_size.y
                     && position
                         .checked_sub(offset)
                         .map(|p| self.scroller.start_drag(p))
@@ -372,6 +370,7 @@ impl<I: ListItem> View for ListView<I> {
                 event: MouseEvent::Release(MouseButton::Left),
                 ..
             } => {
+                log::debug!("releasing scroller");
                 self.scroller.release_grab();
             }
             _ => {