Table: improve performance (#14868)

packages/table/src/table-body.js

 import { getCell, getColumnByCell, getRowIdentity } from './util';
-import { getStyle, hasClass } from 'element-ui/src/utils/dom';
+import { getStyle, hasClass, removeClass, addClass } from 'element-ui/src/utils/dom';
 import ElCheckbox from 'element-ui/packages/checkbox';
 import ElTooltip from 'element-ui/packages/tooltip';
 import debounce from 'throttle-debounce/debounce';
@@ -207,6 +207,23 @@ export default {
     }
   },
 
+  watch: {
+    // don't trigger getter of currentRow in getCellClass. see https://jsfiddle.net/oe2b4hqt/
+    // update DOM manually. see https://github.com/ElemeFE/element/pull/13954/files#diff-9b450c00d0a9dec0ffad5a3176972e40
+    'store.states.hoverRow'(newVal, oldVal) {
+      if (!this.store.states.isComplex) return;
+      const rows = this.$el.querySelectorAll('.el-table__row');
+      const oldRow = rows[oldVal];
+      const newRow = rows[newVal];
+      if (oldRow) {
+        removeClass(oldRow, 'hover-row');
+      }
+      if (newRow) {
+        addClass(newRow, 'hover-row');
+      }
+    }
+  },
+
   data() {
     return {
       tooltipContent: ''
@@ -281,10 +298,6 @@ export default {
         classes.push('current-row');
       }
 
-      if (rowIndex === this.store.states.hoverRow) {
-        classes.push('hover-row');
-      }
-
       if (this.stripe && rowIndex % 2 === 1) {
         classes.push('el-table__row--striped');
       }