Fix using ConnectorResource icons before attaching ComboBox (#10810)

* Fix using ConnectorResource icons before attaching ComboBox

* Update Chrome and PhantomJS screenshots

server/src/main/java/com/vaadin/ui/ComboBox.java

@@ -842,6 +842,10 @@ public class ComboBox<T> extends AbstractSingleSelect<T>
             Resource icon = getItemIconGenerator().apply(value);
             if (icon != null) {
                 if (icon instanceof ConnectorResource) {
+                    if (!isAttached()) {
+                        // Deferred resource generation.
+                        return;
+                    }
                     setResource("selected", icon);
                 }
                 selectedItemIcon = ResourceReference
@@ -851,6 +855,13 @@ public class ComboBox<T> extends AbstractSingleSelect<T>
         getState().selectedItemIcon = selectedItemIcon;
     }
 
+    @Override
+    public void attach() {
+        super.attach();
+
+        updateSelectedItemIcon();
+    }
+
     @Override
     protected Element writeItem(Element design, T item, DesignContext context) {
         Element element = design.appendElement("option");

uitest/src/main/java/com/vaadin/tests/components/combobox/ComboBoxItemIcon.java

@@ -72,8 +72,7 @@ public class ComboBoxItemIcon extends TestBase {
             cb.setItems(IntStream.range(0, 3).boxed());
             cb.setItemIconGenerator(iconGenerator);
 
-            // FIXME: Selecting ConnectorResource on init does not work.
-            // cb.setValue(2);
+            cb.setValue(1);
             addComponent(cb);
         }
     }