fix: memory leak when positioning with popper (#1094)

* fix: memory leak when positioning with popper
* docs: update calculate position docs

Co-authored-by: Jeff <sagalbot@gmail.com>

src/components/Select.vue

@@ -537,9 +537,13 @@
       },
 
       /**
-       * When `appendToBody` is true, this function
-       * is responsible for positioning the drop
-       * down list.
+       * When `appendToBody` is true, this function is responsible for
+       * positioning the drop down list.
+       *
+       * If a function is returned from `calculatePosition`, it will
+       * be called when the drop down list is removed from the DOM.
+       * This allows for any garbage collection you may need to do.
+       *
        * @since v3.7.0
        * @see http://vue-select.org/guide/positioning.html
        */
@@ -551,6 +555,7 @@
          * @param width {string} calculated width in pixels of the dropdown menu
          * @param top {string} absolute position top value in pixels relative to the document
          * @param left {string} absolute position left value in pixels relative to the document
+         * @return {function|void}
          */
         default(dropdownList, component, {width, top, left}) {
           dropdownList.style.top = top;

src/directives/appendToBody.js

@@ -3,7 +3,7 @@ export default {
         if (context.appendToBody) {
             const {height, top, left} = context.$refs.toggle.getBoundingClientRect();
 
-            context.calculatePosition(el, context, {
+            el.unbindPosition = context.calculatePosition(el, context, {
                 width: context.$refs.toggle.clientWidth + 'px',
                 top: (window.scrollY + top + height) + 'px',
                 left: (window.scrollX + left) + 'px',
@@ -13,9 +13,14 @@ export default {
         }
     },
 
-    unbind (el, bindings, vnode) {
-        if (vnode.context.appendToBody && el.parentNode) {
-            el.parentNode.removeChild(el);
+    unbind (el, bindings, {context}) {
+        if (context.appendToBody) {
+            if (el.unbindPosition && typeof el.unbindPosition === 'function') {
+                el.unbindPosition();
+            }
+            if (el.parentNode) {
+                el.parentNode.removeChild(el);
+            }
         }
     },
 }