Merge ed6e863 into 225d182

src/vaadin-combo-box-data-provider-mixin.html

@@ -128,6 +128,21 @@
         return true;
       }
 
+      // Max item count in active range (items cache).
+      // This local var reflects the 'MAX_RANGE_COUNT' in combo box
+      // connector.
+      const MAX_RANGE_COUNT = Math.max(this.pageSize * 2, 500);
+
+      // We skip the page loading when the scrolling has been reset to 0
+      // position after assigning a new items to iron-list, because otherwise
+      // the connector's 'clearPageCallbacks' would be broken and the gaps in
+      // items list occur. Checking items cache size here, since the gaps only
+      // appear when the cache is fully populated and it should be possible for
+      // sure to load the page at the very beginning
+      if (this.$.overlay.resetScrolling && this.filteredItems.length > MAX_RANGE_COUNT) {
+        return false;
+      }
+
       const loadedItem = this.filteredItems[page * this.pageSize];
       if (loadedItem !== undefined) {
         return loadedItem instanceof Vaadin.ComboBoxPlaceholder;

src/vaadin-combo-box-dropdown-wrapper.html

@@ -122,6 +122,15 @@
            */
           theme: String,
 
+          /**
+           * Used to recognize scroller reset after new items have been set
+           * to iron-list and to ignore unwanted pages load
+           */
+          resetScrolling: {
+            type: Boolean,
+            value: false
+          },
+
           _selectedItem: {
             type: Object
           },
@@ -153,12 +162,23 @@
 
           _selector: Object,
 
-          _itemIdPath: String
+          _itemIdPath: String,
+
+          /**
+           * Stores the scroller position before updating the 'items', in
+           * order to restore it immediately after 'items' have been updated
+           */
+          _oldScrollerPosition: {
+            type: Number,
+            value: 0
+          }
         };
       }
 
       static get observers() {
-        return ['_selectorChanged(_selector)', '_loadingChanged(loading)', '_openedChanged(opened, _items, loading)'];
+        return ['_selectorChanged(_selector)', '_loadingChanged(loading)',
+          '_openedChanged(opened, _items, loading)',
+          '_restoreScrollerPosition(_items)'];
       }
 
       _fireTouchAction(sourceEvent) {
@@ -168,7 +188,32 @@
       }
 
       _getItems(opened, items) {
-        return opened ? items : [];
+        if (opened) {
+          if (this._notEmpty(items)) {
+            // iron-list triggers the scroller reset after items update, and
+            // this is no appropriate for undefined size lazy loading.
+            // see https://github.com/vaadin/vaadin-combo-box-flow/issues/386
+            // We store iron-list scrolling position in order to restore
+            // it later on after the items have been updated.
+            this._oldScrollerPosition = this._selector.firstVisibleIndex;
+            this.resetScrolling = true;
+          }
+          return items;
+        }
+        return [];
+      }
+
+      _restoreScrollerPosition(items) {
+        if (this._notEmpty(items)) {
+          // new items size might be less than old scrolling position
+          this._selector.scrollToIndex(Math.min(items.length - 1, this._oldScrollerPosition));
+          this.resetScrolling = false;
+        }
+      }
+
+      _notEmpty(items) {
+        return this._selector !== undefined && items !== undefined &&
+                items.length > 0;
       }
 
       _openedChanged(opened, items, loading) {