scroll bug fix (scroll up then hold for a while, finally release => unpredictable continuous scrolling)

Runtime/widgets/scroll_activity.cs

 
             this._lastDetails = details;
 
-            if (this._retainMomentum && velocity.sign() == this.carriedVelocity.Value.sign()) {
+            if (this._retainMomentum && velocity != 0.0f && velocity.sign() == this.carriedVelocity.Value.sign()) {
                 velocity += this.carriedVelocity.Value;
             }