inertia over synthetic minor fix

Author: dhilt

src/modules/viewport.js

@@ -187,10 +187,10 @@ export default function Viewport(elementRoutines, buffer, element, viewportContr
         topPadding.height(paddingHeight);
         return;
       }
-      const currentPosition = viewport.scrollTop();
-      const newPosition = currentPosition - paddingHeight;
+      const position = viewport.scrollTop();
+      const newPosition = position - paddingHeight;
+      viewport.synthetic = { previous: position, next: newPosition };
       topPadding.height(0);
-      viewport.synthetic = { position: newPosition };
       viewport.scrollTop(newPosition);
     },
 

src/ui-scroll.js

@@ -496,10 +496,17 @@ angular.module('ui.scroll', [])
           if (!viewport.synthetic) {
             return;
           }
-          const position = viewport.synthetic.position;
-          if (viewport.scrollTop() !== position) {
+          const oldPosition = viewport.synthetic.previous;
+          const newPosition = viewport.synthetic.next;
+          if (viewport.scrollTop() !== newPosition) {
             requestAnimationFrame(() => {
-              viewport.scrollTop(position);
+              const position = viewport.scrollTop();
+              const diff = oldPosition - position;
+              if (diff > 0) { // inertia over synthetic
+                viewport.scrollTop(newPosition - diff);
+              } else {
+                viewport.scrollTop(newPosition);
+              }
               viewport.synthetic = null;
             });
             return true;