Fix virtualization after moving the scroller

Author: jods4

src/datagrid.vue

@@ -1,8 +1,7 @@
 <template>
 <div class='dg-wrapper'>
-  <div class='dg-scroller'>
-    <table class='dg' :class='selected && "dg-selectable"'
-          v-virtual='virtual'>
+  <div class='dg-scroller' v-virtual='virtual'>
+    <table class='dg' :class='selected && "dg-selectable"'>
       <thead>
         <tr>
           <th v-if='selected' class='dg-header'>

src/virtual.ts

@@ -51,7 +51,7 @@ type VirtualHTMLTable = HTMLTableElement & { _vscroll: VirtualState };
 const resizeObserver = new ResizeObserver(entries => {
   for (let entry of entries) {
     let table = entry.target as VirtualHTMLTable;
-    table._vscroll.height = entry.contentRect.height - table.tHead!.clientHeight;
+    table._vscroll.height = entry.contentRect.height - table.querySelector('tHead')!.clientHeight;
   }
 });