Add Viewport up/down scroll

src/widget/canvas.rs

@@ -229,6 +229,7 @@ impl Widget<CanvasState> for CanvasWidget {
         let size = ctx.size();
         let rect = size.to_rect();
 
+        ctx.clip(rect);
         ctx.fill(rect, &Color::WHITE);
 
         let page_content_size = data.versioned_canvas.get().content_size();