[ui] Fix single feat. renderer symbol levels reset when changing style (fixes #26547)

python/gui/auto_generated/symbology/qgsrendererwidget.sip.in

@@ -76,6 +76,11 @@ This method should be called whenever the renderer is actually set on the layer.
 Emitted when expression context variables on the associated
 vector layers have been changed. Will request the parent dialog
 to re-synchronize with the variables.
+%End
+
+    void symbolLevelsChanged();
+%Docstring
+Emitted when the symbol levels settings have been changed.
 %End
 
   protected:

python/gui/auto_generated/symbology/qgssymbolselectordialog.sip.in

@@ -71,6 +71,7 @@ Returns the symbol that is currently active in the widget. Can be ``None``.
 :return: The active symbol.
 %End
 
+
   protected:
 
     void loadSymbol();
@@ -78,7 +79,6 @@ Returns the symbol that is currently active in the widget. Can be ``None``.
 Reload the current symbol in the view.
 %End
 
-
     void updateUi();
 %Docstring
 Update the state of the UI based on the currently set symbol layer.

src/gui/symbology/qgsrendererwidget.cpp

@@ -320,7 +320,7 @@ void QgsRendererWidget::showSymbolLevelsDialog( QgsFeatureRenderer *r )
     QgsSymbolLevelsWidget *widget = new QgsSymbolLevelsWidget( r, r->usingSymbolLevels(), panel );
     widget->setPanelTitle( tr( "Symbol Levels" ) );
     connect( widget, &QgsPanelWidget::widgetChanged, widget, &QgsSymbolLevelsWidget::apply );
-    connect( widget, &QgsPanelWidget::widgetChanged, this, &QgsPanelWidget::widgetChanged );
+    connect( widget, &QgsPanelWidget::widgetChanged, [ = ]() { emit widgetChanged(); emit symbolLevelsChanged(); } );
     panel->openPanel( widget );
     return;
   }
@@ -329,6 +329,7 @@ void QgsRendererWidget::showSymbolLevelsDialog( QgsFeatureRenderer *r )
   if ( dlg.exec() )
   {
     emit widgetChanged();
+    emit symbolLevelsChanged();
   }
 }
 

src/gui/symbology/qgsrendererwidget.h

@@ -88,6 +88,11 @@ class GUI_EXPORT QgsRendererWidget : public QgsPanelWidget
      */
     void layerVariablesChanged();
 
+    /**
+     * Emitted when the symbol levels settings have been changed.
+     */
+    void symbolLevelsChanged();
+
   protected:
     QgsVectorLayer *mLayer = nullptr;
     QgsStyle *mStyle = nullptr;

src/gui/symbology/qgssinglesymbolrendererwidget.cpp

@@ -57,6 +57,12 @@ QgsSingleSymbolRendererWidget::QgsSingleSymbolRendererWidget( QgsVectorLayer *la
   mSelector = new QgsSymbolSelectorWidget( mSingleSymbol, mStyle, mLayer, nullptr );
   connect( mSelector, &QgsSymbolSelectorWidget::symbolModified, this, &QgsSingleSymbolRendererWidget::changeSingleSymbol );
   connect( mSelector, &QgsPanelWidget::showPanel, this, &QgsPanelWidget::openPanel );
+  connect( this, &QgsRendererWidget::symbolLevelsChanged, [ = ]()
+  {
+    delete mSingleSymbol;
+    mSingleSymbol = mRenderer->symbol()->clone();
+    mSelector->loadSymbol( mSingleSymbol );
+  } );
 
   QVBoxLayout *layout = new QVBoxLayout( this );
   layout->setContentsMargins( 0, 0, 0, 0 );

src/gui/symbology/qgssymbolselectordialog.cpp

@@ -385,6 +385,13 @@ QgsSymbolWidgetContext QgsSymbolSelectorWidget::context() const
 
 void QgsSymbolSelectorWidget::loadSymbol( QgsSymbol *symbol, SymbolLayerItem *parent )
 {
+  if ( !parent )
+  {
+    mSymbol = symbol;
+    model->clear();
+    parent = static_cast<SymbolLayerItem *>( model->invisibleRootItem() );
+  }
+
   SymbolLayerItem *symbolItem = new SymbolLayerItem( symbol );
   QFont boldFont = symbolItem->font();
   boldFont.setBold( true );

src/gui/symbology/qgssymbolselectordialog.h

@@ -127,6 +127,14 @@ class GUI_EXPORT QgsSymbolSelectorWidget: public QgsPanelWidget, private Ui::Qgs
      */
     QgsSymbol *symbol() { return mSymbol; }
 
+    /**
+     * Load the given symbol into the widget.
+     * \param symbol The symbol to load.
+     * \param parent The parent symbol layer item. If the parent parameter is null, the whole symbol and model will be reset.
+     * \note not available in Python bindings
+     */
+    void loadSymbol( QgsSymbol *symbol, SymbolLayerItem *parent = nullptr ) SIP_SKIP;
+
   protected:
 
     /**
@@ -134,14 +142,6 @@ class GUI_EXPORT QgsSymbolSelectorWidget: public QgsPanelWidget, private Ui::Qgs
      */
     void loadSymbol();
 
-    /**
-     * Load the given symbol into the widget.
-     * \param symbol The symbol to load.
-     * \param parent The parent symbol layer item.
-     * \note not available in Python bindings
-     */
-    void loadSymbol( QgsSymbol *symbol, SymbolLayerItem *parent ) SIP_SKIP;
-
     /**
      * Update the state of the UI based on the currently set symbol layer.
      */