fix: change calculation method of element on scroll

src/components/SideMenu.vue

@@ -49,9 +49,12 @@ function onScroll() {
     if (!v.element) v.element = document.getElementById(v.anchor) || undefined;
     if (!v.element) return;
 
+    const containerRect = container.getBoundingClientRect();
+    const elementRect = v.element.getBoundingClientRect();
+
     if (
-      container.scrollTop + container.clientHeight / 2 >
-      v.element.offsetTop
+      containerRect.y + containerRect.height / 2 >=
+      elementRect.y + elementRect.height / 2
     ) {
       current = v.anchor;
     }