made drag offset optional

Author: cameron-toy

packages/dragdrop/src/index.ts

@@ -828,12 +828,18 @@ namespace Private {
 
   /**
    * Find the drag scroll target under the mouse, if any.
+   * 
+   * @param event - The mouse event related to the action.
+   * 
+   * @param dragOffsetX - the number of pixels to offset the drag in the x direction.
+   * 
+   * @param dragOffsetY - the number of pixels to offset the drag in the y direction.
    */
   export
-  function findScrollTarget(event: MouseEvent, dragOffsetX: number, dragOffsetY: number): IScrollTarget | null {
+  function findScrollTarget(event: MouseEvent, dragOffsetX?: number, dragOffsetY?: number): IScrollTarget | null {
     // Look up the client mouse position.
-    const x = event.clientX + dragOffsetX;
-    const y = event.clientY + dragOffsetY;
+    const x = event.clientX + (dragOffsetX !== undefined ? dragOffsetX : 0);
+    const y = event.clientY + (dragOffsetY !== undefined ? dragOffsetY : 0);
 
     // Get the element under the mouse.
     let element: Element | null = document.elementFromPoint(x, y);