fix: defer scrollToIndex calls until columnTree is available (#2254)

Author: vursen

src/vaadin-grid-data-provider-mixin.html

@@ -509,7 +509,7 @@
 
     scrollToIndex(index) {
       super.scrollToIndex(index);
-      if (!isNaN(index) && (this._cache.isLoading() || !this.clientHeight)) {
+      if (!isNaN(index) && (this._cache.isLoading() || !this.clientHeight || !this._columnTree)) {
         this.__pendingScrollToIndex = index;
       }
     }

test/scroll-to-index.html

@@ -48,6 +48,7 @@
       </vaadin-grid>
     </template>
   </test-fixture>
+
   <script>
     describe('Scroll to index', () => {
 
@@ -318,7 +319,23 @@
         grid.scrollToIndex(14);
       });
     });
+
+    describe('before grid is fully initialized', () => {
+      let grid;
+
+      beforeEach((done) => {
+        grid = fixture('small');
+        grid.items = Array.from({length: 100}, (_, index) => `Item ${index}`);
+        grid.scrollToIndex(50);
+        flushGrid(grid);
+        animationFrameFlush(done);
+      });
+
+      it('should scroll to index after items are rendered', () => {
+        expect(grid._firstVisibleIndex).to.equal(50);
+      });
+    });
   </script>
 </body>
 
-</html>
+</html>