drag by pan fixed y position.

lib/animation-timeline.js

@@ -2077,7 +2077,7 @@ var timeline_Timeline = /*#__PURE__*/function (_TimelineEventsEmitte) {
         this._scrollContainer.scrollLeft = newLeft;
       }
 
-      this._scrollContainer.scrollTop = Math.round(start.y - pos.y);
+      this._scrollContainer.scrollTop = Math.round(scrollStartPos.y + start.y - pos.y);
     }
   }, {
     key: "_scrollBySelectionOutOfBounds",

src/timeline.ts

@@ -841,7 +841,7 @@ export class Timeline extends TimelineEventsEmitter {
     } else {
       this._scrollContainer.scrollLeft = newLeft;
     }
-    this._scrollContainer.scrollTop = Math.round(start.y - pos.y);
+    this._scrollContainer.scrollTop = Math.round(scrollStartPos.y + start.y - pos.y);
   }
 
   _scrollBySelectionOutOfBounds(pos: DOMPoint): boolean {