[3d] Fix unnecessary terrain map updates when changing 3D renderer

When using layer styling dock, every change in 3D rendering configuration
was also triggering update of 2D map which also forces update of all
terrain tile textures with a new 2D map which wasn't really needed.
The fix makes the triggerRefresh() call on layer optional - each layer
styling dock config widget can tell whether its updates require 2D map
refresh (true by default).

python/gui/auto_generated/qgsmaplayerconfigwidget.sip.in

@@ -36,6 +36,15 @@ A panel widget that can be shown in the map style dock
    Keep the loading light as possible for speed in the UI.
 %End
 
+    virtual bool shouldTriggerLayerRepaint() const;
+%Docstring
+Whether this config widget changes map layer properties in a way that triggerRepaint() should
+be called for the layer after applying changes. This is true by default, but some config widgets
+(for example 3D rendering config) do not need layer repaint as they do not modify 2D map rendering.
+
+.. versionadded:: 3.8
+%End
+
   public slots:
 
     virtual void apply() = 0;

src/app/3d/qgsvectorlayer3drendererwidget.h

@@ -66,6 +66,9 @@ class QgsVectorLayer3DRendererWidget : public QgsMapLayerConfigWidget
 
     void setDockMode( bool dockMode ) override;
 
+    //! Only modifies 3D renderer so we do not want layer repaint (which would trigger unnecessary terrain map update)
+    bool shouldTriggerLayerRepaint() const override { return false; }
+
   public slots:
     void apply() override;
 

src/app/qgslayerstylingwidget.cpp

@@ -271,6 +271,7 @@ void QgsLayerStylingWidget::apply()
   QWidget *current = mWidgetStack->mainPanel();
 
   bool styleWasChanged = false;
+  bool triggerRepaint = false;  // whether the change needs the layer to be repainted
   if ( QgsLabelingWidget *widget = qobject_cast<QgsLabelingWidget *>( current ) )
   {
     widget->apply();
@@ -286,31 +287,34 @@ void QgsLayerStylingWidget::apply()
       QgsRendererAbstractMetadata *m = QgsApplication::rendererRegistry()->rendererMetadata( layer->renderer()->type() );
       undoName = QStringLiteral( "Style Change - %1" ).arg( m->visibleName() );
       styleWasChanged = true;
+      triggerRepaint = true;
     }
   }
   else if ( QgsRasterTransparencyWidget *widget = qobject_cast<QgsRasterTransparencyWidget *>( current ) )
   {
     widget->apply();
     styleWasChanged = true;
+    triggerRepaint = true;
   }
   else if ( qobject_cast<QgsRasterHistogramWidget *>( current ) )
   {
     mRasterStyleWidget->apply();
     styleWasChanged = true;
+    triggerRepaint = true;
   }
   else if ( QgsMapLayerConfigWidget *widget = qobject_cast<QgsMapLayerConfigWidget *>( current ) )
   {
     widget->apply();
     styleWasChanged = true;
+    triggerRepaint = widget->shouldTriggerLayerRepaint();
   }
 
-  pushUndoItem( undoName );
+  pushUndoItem( undoName, triggerRepaint );
 
   if ( styleWasChanged )
   {
     emit styleChanged( mCurrentLayer );
     QgsProject::instance()->setDirty( true );
-    mCurrentLayer->triggerRepaint();
   }
   connect( mCurrentLayer, &QgsMapLayer::styleChanged, this, &QgsLayerStylingWidget::updateCurrentWidgetLayer );
 }
@@ -608,25 +612,26 @@ void QgsLayerStylingWidget::liveApplyToggled( bool value )
   settings.setValue( QStringLiteral( "UI/autoApplyStyling" ), value );
 }
 
-void QgsLayerStylingWidget::pushUndoItem( const QString &name )
+void QgsLayerStylingWidget::pushUndoItem( const QString &name, bool triggerRepaint )
 {
   QString errorMsg;
   QDomDocument doc( QStringLiteral( "style" ) );
   QDomElement rootNode = doc.createElement( QStringLiteral( "qgis" ) );
   doc.appendChild( rootNode );
   mCurrentLayer->writeStyle( rootNode, doc, errorMsg, QgsReadWriteContext() );
-  mCurrentLayer->undoStackStyles()->push( new QgsMapLayerStyleCommand( mCurrentLayer, name, rootNode, mLastStyleXml ) );
+  mCurrentLayer->undoStackStyles()->push( new QgsMapLayerStyleCommand( mCurrentLayer, name, rootNode, mLastStyleXml, triggerRepaint ) );
   // Override the last style on the stack
   mLastStyleXml = rootNode.cloneNode();
 }
 
 
-QgsMapLayerStyleCommand::QgsMapLayerStyleCommand( QgsMapLayer *layer, const QString &text, const QDomNode &current, const QDomNode &last )
+QgsMapLayerStyleCommand::QgsMapLayerStyleCommand( QgsMapLayer *layer, const QString &text, const QDomNode &current, const QDomNode &last, bool triggerRepaint )
   : QUndoCommand( text )
   , mLayer( layer )
   , mXml( current )
   , mLastState( last )
   , mTime( QTime::currentTime() )
+  , mTriggerRepaint( triggerRepaint )
 {
 }
 
@@ -635,15 +640,17 @@ void QgsMapLayerStyleCommand::undo()
   QString error;
   QgsReadWriteContext context = QgsReadWriteContext();
   mLayer->readStyle( mLastState, error, context );
-  mLayer->triggerRepaint();
+  if ( mTriggerRepaint )
+    mLayer->triggerRepaint();
 }
 
 void QgsMapLayerStyleCommand::redo()
 {
   QString error;
   QgsReadWriteContext context = QgsReadWriteContext();
   mLayer->readStyle( mXml, error, context );
-  mLayer->triggerRepaint();
+  if ( mTriggerRepaint )
+    mLayer->triggerRepaint();
 }
 
 bool QgsMapLayerStyleCommand::mergeWith( const QUndoCommand *other )
@@ -664,6 +671,7 @@ bool QgsMapLayerStyleCommand::mergeWith( const QUndoCommand *other )
 
   mXml = otherCmd->mXml;
   mTime = otherCmd->mTime;
+  mTriggerRepaint |= otherCmd->mTriggerRepaint;
   return true;
 }
 

src/app/qgslayerstylingwidget.h

@@ -56,7 +56,7 @@ class APP_EXPORT QgsLayerStyleManagerWidgetFactory : public QgsMapLayerConfigWid
 class APP_EXPORT QgsMapLayerStyleCommand : public QUndoCommand
 {
   public:
-    QgsMapLayerStyleCommand( QgsMapLayer *layer, const QString &text, const QDomNode &current, const QDomNode &last );
+    QgsMapLayerStyleCommand( QgsMapLayer *layer, const QString &text, const QDomNode &current, const QDomNode &last, bool triggerRepaint = true );
 
     /**
      * Returns unique ID for this kind of undo command.
@@ -75,6 +75,7 @@ class APP_EXPORT QgsMapLayerStyleCommand : public QUndoCommand
     QDomNode mXml;
     QDomNode mLastState;
     QTime mTime;
+    bool mTriggerRepaint = true;
 };
 
 class APP_EXPORT QgsLayerStylingWidget : public QWidget, private Ui::QgsLayerStylingWidgetBase
@@ -129,7 +130,7 @@ class APP_EXPORT QgsLayerStylingWidget : public QWidget, private Ui::QgsLayerSty
     void liveApplyToggled( bool value );
 
   private:
-    void pushUndoItem( const QString &name );
+    void pushUndoItem( const QString &name, bool triggerRepaint = true );
     int mNotSupportedPage;
     int mLayerPage;
     QTimer *mAutoApplyTimer = nullptr;

src/gui/qgsmaplayerconfigwidget.h

@@ -45,6 +45,14 @@ class GUI_EXPORT QgsMapLayerConfigWidget : public QgsPanelWidget
        */
     QgsMapLayerConfigWidget( QgsMapLayer *layer, QgsMapCanvas *canvas, QWidget *parent = nullptr );
 
+    /**
+     * Whether this config widget changes map layer properties in a way that triggerRepaint() should
+     * be called for the layer after applying changes. This is true by default, but some config widgets
+     * (for example 3D rendering config) do not need layer repaint as they do not modify 2D map rendering.
+     * \since QGIS 3.8
+     */
+    virtual bool shouldTriggerLayerRepaint() const { return true; }
+
   public slots:
 
     /**