clamp offset when show on screen

Runtime/rendering/viewport.cs

 
             D.assert(targetOffset != null);
 
-            if (duration == TimeSpan.Zero) {
-                offset.jumpTo(targetOffset.offset);
-            }
-            else {
-                offset.animateTo(targetOffset.offset, duration: duration.Value, curve: curve);
-            }
-
+            offset.moveTo(targetOffset.offset, duration: duration.Value, curve: curve);
             return targetOffset.rect;
         }
     }

Runtime/rendering/viewport_offset.cs

 
         public abstract IPromise animateTo(float to, TimeSpan duration, Curve curve);
 
+        public virtual IPromise moveTo(float to, TimeSpan? duration, Curve curve = null, bool clamp = true) {
+            if (duration == null || duration.Value == TimeSpan.Zero) {
+                this.jumpTo(to);
+                return Promise.Resolved();
+            } else {
+                return this.animateTo(to, duration: duration??TimeSpan.Zero , curve: curve ?? Curves.ease);
+            }
+        }
+
         public abstract ScrollDirection userScrollDirection { get; }
 
         public abstract bool allowImplicitScrolling { get; }

Runtime/widgets/scroll_position.cs

 
         public readonly ValueNotifier<bool> isScrollingNotifier = new ValueNotifier<bool>(false);
 
+        public override IPromise moveTo(float to, TimeSpan? duration, Curve curve = null, bool clamp = true) {
+            if (clamp)
+                to = to.clamp(this.minScrollExtent, this.maxScrollExtent);
+
+            return base.moveTo(to, duration: duration, curve: curve, clamp: clamp);
+        }
+        
         public override bool allowImplicitScrolling {
             get { return this.physics.allowImplicitScrolling; }
         }