fix issues 24, issues 25 (#26)

* fix #24

* fix #25

Author: gaodeng

src/VirtualGrid.vue

@@ -1,5 +1,5 @@
 <script lang="ts">
-import { Prop, Component, Vue, ProvideReactive } from 'vue-property-decorator';
+import { Prop, Component, Watch, Vue, ProvideReactive } from 'vue-property-decorator';
 import {
     getGridGapDefault,
     getColumnCountDefault,
@@ -50,7 +50,9 @@ interface RenderData<P> {
     firstRenderedRowOffset: number | null;
 }
 
-@Component
+@Component({
+    name: 'VirtualGrid',
+})
 export default class VirtualGrid<P> extends Vue {
     @Prop({ required: true }) items: Item<P>[];
     @Prop({ default: () => () => true }) updateFunction: () => Promise<boolean>;
@@ -62,6 +64,7 @@ export default class VirtualGrid<P> extends Vue {
         width: number,
         columnWidth: number
     ) => number;
+    @Prop({ default: null }) scrollElement: HTMLElement | null;
     @Prop({ default: 500 }) updateTriggerMargin: number;
     @Prop({ default: null }) loader: Vue.Component;
     @Prop({ default: false }) debug: boolean;
@@ -99,12 +102,18 @@ export default class VirtualGrid<P> extends Vue {
         this.ref = this.$refs.virtualGrid as Element;
         this.initiliazeGrid();
         window.addEventListener('resize', this.resize);
-        window.addEventListener('scroll', this.scroll);
+        (this.scrollElement ?? window).addEventListener('scroll', this.scroll);
     }
 
     beforeDestroy() {
         window.removeEventListener('resize', this.resize);
-        window.removeEventListener('scroll', this.scroll);
+        (this.scrollElement ?? window).removeEventListener('scroll', this.scroll);
+    }
+
+    @Watch('scrollElement')
+    onScrollElementChanged(scrollElement: HTMLElement | null, oldScrollElement: HTMLElement | null) {
+        (oldScrollElement ?? window).removeEventListener('scroll', this.scroll);
+        (scrollElement ?? window).addEventListener('scroll', this.scroll);
     }
 
     resize(): void {
@@ -394,6 +403,7 @@ export default class VirtualGrid<P> extends Vue {
     getElementOffset(element: Element) {
         return window.scrollY + element.getBoundingClientRect().top;
     }
+
 }
 </script>
 
@@ -424,6 +434,7 @@ export default class VirtualGrid<P> extends Vue {
             <div
                 v-for="item in renderData.cellsToRender"
                 :key="item.id"
+                class="grid-item-wrapper"
                 :style="{
                     'height': `${item.height}px`,
                     'grid-column-start': item.columnNumber,