Fix unnecessary layer reloads triggering after project load, which

also causes a leak

python/gui/auto_generated/qgsmapcanvas.sip.in

@@ -728,9 +728,27 @@ Repaints the canvas map
 
     void refreshAllLayers();
 %Docstring
-Reload all layers, clear the cache and refresh the canvas
+Reload all layers (including refreshing layer properties from their data sources),
+clears the cache and refreshes the canvas.
+
+.. note::
+
+   Consider using the less expensive redrawAllLayers() method if a layer reload
+   from the data provider is not required.
 
 .. versionadded:: 2.9
+%End
+
+    void redrawAllLayers();
+%Docstring
+Clears all cached images and redraws all layers.
+
+.. note::
+
+   Unlike refreshAllLayers(), this does NOT reload layers themselves, and accordingly
+   is more "lightweight". Use this method when only an update of the layer's renderers is required.
+
+.. versionadded:: 3.10
 %End
 
     void selectionChangedSlot();

src/app/layout/qgslayoutdesignerdialog.cpp

@@ -4308,7 +4308,7 @@ void QgsLayoutDesignerDialog::atlasFeatureChanged( const QgsFeature &feature )
   mapCanvas->expressionContextScope().addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "atlas_featureid" ), feature.id(), true ) );
   mapCanvas->expressionContextScope().addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "atlas_geometry" ), QVariant::fromValue( feature.geometry() ), true ) );
   mapCanvas->stopRendering();
-  mapCanvas->refreshAllLayers();
+  mapCanvas->redrawAllLayers();
 
   mView->setSectionLabel( atlas->nameForPage( atlas->currentFeatureNumber() ) );
 }

src/app/qgsstatusbarcoordinateswidget.cpp

@@ -280,7 +280,7 @@ void QgsStatusBarCoordinatesWidget::refreshMapCanvas()
 
   //stop any current rendering
   mMapCanvas->stopRendering();
-  mMapCanvas->refreshAllLayers();
+  mMapCanvas->redrawAllLayers();
 }
 
 void QgsStatusBarCoordinatesWidget::showMouseCoordinates( const QgsPointXY &p )

src/gui/qgsmapcanvas.cpp

@@ -154,10 +154,10 @@ QgsMapCanvas::QgsMapCanvas( QWidget *parent )
   } );
 
   // refresh canvas when a remote svg/image has finished downloading
-  connect( QgsApplication::svgCache(), &QgsSvgCache::remoteSvgFetched, this, &QgsMapCanvas::refreshAllLayers );
-  connect( QgsApplication::imageCache(), &QgsImageCache::remoteImageFetched, this, &QgsMapCanvas::refreshAllLayers );
+  connect( QgsApplication::svgCache(), &QgsSvgCache::remoteSvgFetched, this, &QgsMapCanvas::redrawAllLayers );
+  connect( QgsApplication::imageCache(), &QgsImageCache::remoteImageFetched, this, &QgsMapCanvas::redrawAllLayers );
   // refresh canvas when project color scheme is changed -- if layers use project colors, they need to be redrawn
-  connect( QgsProject::instance(), &QgsProject::projectColorsChanged, this, &QgsMapCanvas::refreshAllLayers );
+  connect( QgsProject::instance(), &QgsProject::projectColorsChanged, this, &QgsMapCanvas::redrawAllLayers );
 
   //segmentation parameters
   QgsSettings settings;
@@ -2211,6 +2211,11 @@ void QgsMapCanvas::refreshAllLayers()
     layer->reload();
   }
 
+  redrawAllLayers();
+}
+
+void QgsMapCanvas::redrawAllLayers()
+{
   // clear the cache
   clearCache();
 

src/gui/qgsmapcanvas.h

@@ -645,11 +645,26 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
     void refresh();
 
     /**
-     * Reload all layers, clear the cache and refresh the canvas
+     * Reload all layers (including refreshing layer properties from their data sources),
+     * clears the cache and refreshes the canvas.
+     *
+     * \note Consider using the less expensive redrawAllLayers() method if a layer reload
+     * from the data provider is not required.
+     *
      * \since QGIS 2.9
      */
     void refreshAllLayers();
 
+    /**
+     * Clears all cached images and redraws all layers.
+     *
+     * \note Unlike refreshAllLayers(), this does NOT reload layers themselves, and accordingly
+     * is more "lightweight". Use this method when only an update of the layer's renderers is required.
+     *
+     * \since QGIS 3.10
+     */
+    void redrawAllLayers();
+
     //! Receives signal about selection change, and pass it on with layer info
     void selectionChangedSlot();