Fix rowClasses changes potentially wiping out --striped classes

+ switch to `useUpdateEffect` for extra microperf, since `rowClasses` gets set on creation

src/components/datagrid/body/data_grid_row_manager.test.ts

@@ -165,5 +165,13 @@ describe('row manager', () => {
       expect(row0.classList.contains('world')).toBe(false);
       expect(row1.classList.contains('test')).toBe(true);
     });
+
+    it('correctly preserves EUI classes when adding/removing classes dynamically', () => {
+      rerender({ ...mockArgs, rowClasses: undefined });
+
+      expect(getRow(0).classList.value).toEqual(
+        'euiDataGridRow euiDataGridRow--striped'
+      );
+    });
   });
 });

src/components/datagrid/body/data_grid_row_manager.ts

@@ -6,7 +6,9 @@
  * Side Public License, v 1.
  */
 
-import { useRef, useCallback, useEffect, RefObject } from 'react';
+import { useRef, useCallback, RefObject } from 'react';
+import { useUpdateEffect } from '../../../services';
+
 import { EuiDataGridRowManager, EuiDataGridStyle } from '../data_grid_types';
 
 export const useRowManager = ({
@@ -76,13 +78,17 @@ export const useRowManager = ({
   );
 
   // Update row classes dynamically whenever a new prop is passed in
-  useEffect(() => {
+  useUpdateEffect(() => {
     if (rowClasses) {
       rowIdToElements.current.forEach((rowElement, rowIndex) => {
+        const euiClasses = Array.from(rowElement.classList)
+          .filter((className) => className.startsWith('euiDataGridRow'))
+          .join(' ');
+
         if (rowClasses[rowIndex]) {
-          rowElement.classList.value = `euiDataGridRow ${rowClasses[rowIndex]}`;
+          rowElement.classList.value = `${euiClasses} ${rowClasses[rowIndex]}`;
         } else {
-          rowElement.classList.value = 'euiDataGridRow'; // Clear any added classes
+          rowElement.classList.value = euiClasses; // Clear any added classes
         }
       });
     }