WT-11868: make new signal work with lazy loading

Make currentWidgetChanged() signal work correcly when using WMenuItem
with the lazy loading policy.

Author: Romain Mardulyn

src/Wt/WMenu.C

@@ -230,13 +230,7 @@ WMenuItem *WMenu::insertItem(int index, std::unique_ptr<WMenuItem> item)
       contentsStack_->setLoadPolicy(contentsStack_->count()-1, result->loadPolicy_);
 
       if (contentsStack_->count() == 1) {
-        setCurrent(0);
-        if (loaded()) {
-          currentItem()->loadContents();
-        }
-        contentsStack_->setCurrentWidget(contents);
-
-        renderSelected(result, true);
+        select(0, false);
       } else
         renderSelected(result, false);
     } else
@@ -308,12 +302,24 @@ void WMenu::select(int index, bool changePath)
 
   selectVisual(current_, changePath, true);
 
+  WMenuItem *item;
+
   if (index != -1) {
-    WMenuItem *item = itemAt(index);
+    item = itemAt(index);
     item->show();
-    if (loaded())
+    if (loaded()) {
+      bool itemLoaded = item->contentsLoaded();
       item->loadContents();
 
+      //if the item was not loaded before selectVisual we emit the signal
+      if (!itemLoaded && 
+        contentsStack_ && 
+        contentsStack_->loadPolicies_[contentsStack_->currentIndex_] == ContentLoading::Lazy) {
+        WContainerWidget* container = dynamic_cast<WContainerWidget*>(contentsStack_->currentWidget());
+        contentsStack_->currentWidgetChanged().emit(container->widget(0));
+      }
+    }
+
     observing_ptr<WMenu> self = this;
 
     if (changePath && emitPathChange_) {
@@ -515,8 +521,9 @@ void WMenu::undoSelectVisual()
     app->setInternalPath(prevPath);
   }
 
-  if (contentsStack_)
+  if (contentsStack_) {
     contentsStack_->setCurrentIndex(prevStackIndex);
+  }
 }
 
 WMenuItem *WMenu::currentItem() const

src/Wt/WMenu.h

@@ -331,8 +331,13 @@ class WT_API WMenu : public WCompositeWidget
    *
    * Inserts a menu item. Use this form to insert specialized WMenuItem
    * implementations.
+   * 
+   * \note When using the WMenu with a WStackedWidget, the first WMenuItem 
+   *       added will be automatically selected. This implies that this
+   *       function can trigger the itemSelectRendered() and 
+   *       itemSelected() signals.
    *
-   * \sa insertItem(WMenuItem *item)
+   * \sa insertItem(WMenuItem *item), itemSelected(), itemSelectRendered()
    */
   virtual WMenuItem *insertItem(int index, std::unique_ptr<WMenuItem> item);
 

src/Wt/WStackedWidget.C

@@ -101,7 +101,7 @@ void WStackedWidget::setCurrentIndex(int index, const WAnimation& animation,
                                      bool autoReverse)
 {
   if (!animation.empty() &&
-      WApplication::instance()->environment().supportsCss3Animations() &&
+    WApplication::instance()->environment().supportsCss3Animations() &&
       ((isRendered() && javaScriptDefined_) || !canOptimizeUpdates())) {
     if (canOptimizeUpdates() && index == currentIndex_)
       return;
@@ -133,7 +133,17 @@ void WStackedWidget::setCurrentIndex(int index, const WAnimation& animation,
       doJavaScript(jsRef() + ".wtObj.setCurrent("
                    + widget(currentIndex_)->jsRef() + ");");
   }
-  currentWidgetChanged().emit(currentWidget());
+
+  if (loadPolicies_[currentIndex_] == ContentLoading::Lazy) {
+    WContainerWidget* container = dynamic_cast<WContainerWidget*>(currentWidget());
+
+    // If the container is empty, the content is not yet loaded. We rely on the function calling this one to emit the signal.
+    if (container->count()) {
+      currentWidgetChanged().emit(container->widget(0)); 
+    }
+  } else {
+    currentWidgetChanged().emit(currentWidget());
+  }
 }
 
 void WStackedWidget::loadAnimateJS()