fix document scroll issue

DragSelect/src/methods/scrollElement.ts

@@ -1,21 +1,36 @@
 import { DSArea, DSEdges } from "../types"
 
+let DOC_ELEMENT: HTMLElement; // we store this in the module cache as it is highly unlikely the user to change document
+
 /** Scroll the element in the specified direction */
 export const scrollElement = (element?: DSArea, directions?: DSEdges, multiplier: number = 1) => {
   if (!directions?.length || !element) return
 
-  const docEl =
-    Number.isFinite(document?.documentElement?.scrollTop) && document.documentElement
-  const _element =
-    element instanceof Document ? docEl || document.body : element
+  let _element = element as HTMLElement | SVGElement; // we are typecasting because if it's of type Document we grab the corresponding HTMLElement anyhow
+  if(element instanceof Document) {
+    if(DOC_ELEMENT) _element = DOC_ELEMENT; // cached
+    // either the document is scrollable or the body is scrollable
+    if(Number.isFinite(document?.documentElement?.scrollTop)) {
+      const prev = document.documentElement.scrollTop;
+      document.documentElement.scrollTop += 1
+      if (document.documentElement.scrollTop === prev) {
+        _element = document.body;
+        DOC_ELEMENT = document.body;
+      } else {
+        document.documentElement.scrollTop = prev;
+        _element = document.documentElement;
+        DOC_ELEMENT = document.documentElement;
+      }
+    }
+  }
 
   const scrollTop = directions.includes('top') && _element.scrollTop > 0
   const scrollBot =
     directions.includes('bottom') && _element.scrollTop < _element.scrollHeight
   const scrollLeft = directions.includes('left') && _element.scrollLeft > 0
   const scrollRight =
     directions.includes('right') && _element.scrollLeft < _element.scrollWidth
-    
+
   if (scrollTop) _element.scrollTop -= 1 * multiplier
   if (scrollBot) _element.scrollTop += 1 * multiplier
   if (scrollLeft) _element.scrollLeft -= 1 * multiplier