Picker layout fix (WI:3391028) (#2584)

* Forcing layout after adding new items to Picker

* Suggested code review change.

vnext/ReactUWP/Views/PickerViewManager.cpp

@@ -35,13 +35,16 @@ class PickerShadowNode : public ShadowNodeBase
   PickerShadowNode();
   void createView() override;
   void updateProperties(const folly::dynamic&& props) override;
+  bool IsExternalLayoutDirty() const override { return m_hasNewItems; }
+  void DoExtraLayoutPrep(YGNodeRef yogaNode) override;
 
 private:
   void RepopulateItems();
   static void OnSelectionChanged(IReactInstance& instance, int64_t tag, folly::dynamic&& value, int32_t selectedIndex, folly::dynamic&& text);
 
   folly::dynamic m_items;
   int32_t m_selectedIndex = -1;
+  bool m_hasNewItems = false;
 
   // FUTURE: remove when we can require RS5+
   bool m_isEditableComboboxSupported;
@@ -53,6 +56,17 @@ PickerShadowNode::PickerShadowNode()
   m_isEditableComboboxSupported = winrt::Windows::Foundation::Metadata::ApiInformation::IsPropertyPresent(L"Windows.UI.Xaml.Controls.ComboBox", L"IsEditableProperty");
 }
 
+void PickerShadowNode::DoExtraLayoutPrep(YGNodeRef yogaNode)
+{
+  if (!m_hasNewItems)
+    return;
+
+  m_hasNewItems = false;
+
+  auto comboBox = GetView().try_as<winrt::ComboBox>();
+  comboBox.UpdateLayout();
+}
+
 void PickerShadowNode::createView()
 {
   Super::createView();
@@ -158,6 +172,7 @@ void PickerShadowNode::RepopulateItems()
         comboboxItem.Foreground(BrushFrom(item["textColor"]));
       comboBoxItems.Append(comboboxItem);
     }
+    m_hasNewItems = true;
   }
   if (m_selectedIndex < static_cast<int32_t>(m_items.size()))
     combobox.SelectedIndex(m_selectedIndex);