qgisapp: Update raster layer style when render details have changed

`QgsRasterLayer` updates the GUI (the renderer widget and the legend)
when the rendered values are updated. This does not work when multiple
canvas are displayed because each canvas causes a renderer update.

This issue is fixed by removing the GUI update from the raster layer
code. With this change, when the rendered values are updated, these
statistics are propagated to the canvas by the use of a
`QgsRenderedItemDetails`: `QgsRenderedCalculatedResults`. Then, only
the main canvas is notified of this change and updates the GUI
accordingly.

src/app/qgisapp.cpp

@@ -446,6 +446,8 @@
 #include "devtools/querylogger/qgsappquerylogger.h"
 #include "devtools/querylogger/qgsqueryloggerwidgetfactory.h"
 #include "devtools/profiler/qgsprofilerwidgetfactory.h"
+#include "qgsrendereditemresults.h"
+#include "qgsrenderedcalculatedresults.h"
 
 #include "browser/qgsinbuiltdataitemproviders.h"
 
@@ -1071,6 +1073,7 @@ QgisApp::QgisApp( QSplashScreen *splash, bool restorePlugins, bool skipBadLayers
   mMapCanvas = new QgsMapCanvas( centralWidget );
   mMapCanvas->setObjectName( QStringLiteral( "theMapCanvas" ) );
   mMapCanvas->setFlags( Qgis::MapCanvasFlag::ShowMainAnnotationLayer );
+  connect( mMapCanvas, &QgsMapCanvas::calculatedRenderDetailsChanged, this, &QgisApp::onCalculatedRenderDetailsChanged );
 
   // before anything, let's freeze canvas redraws
   QgsCanvasRefreshBlocker refreshBlocker;
@@ -17588,3 +17591,38 @@ void QgisApp::showEvent( QShowEvent *event )
     }
   } );
 }
+
+void QgisApp::onCalculatedRenderDetailsChanged()
+{
+  const QgsRenderedItemResults *renderedItemResults = mMapCanvas->renderedItemResults( false );
+  if ( !renderedItemResults )
+  {
+    return;
+  }
+
+  const QList<QgsRenderedItemDetails *> items = renderedItemResults->renderedItems();
+  QList<QString> computedLayerIds;
+  for ( const QgsRenderedItemDetails *item : renderedItemResults->renderedItems() )
+  {
+    if ( const QgsRenderedCalculatedResults *calculatedResults = dynamic_cast< const QgsRenderedCalculatedResults *>( item ) )
+    {
+      computedLayerIds.append( calculatedResults->layerId() );
+    }
+  }
+
+  if ( !computedLayerIds.empty() )
+  {
+    for ( QgsMapLayer *layer : mMapCanvas->layers() )
+    {
+      // refresh the legend and style of the raster layer
+      if ( QgsRasterLayer *rasterLayer = qobject_cast<QgsRasterLayer *>( layer ) )
+      {
+        if ( computedLayerIds.contains( rasterLayer->id() ) )
+        {
+          rasterLayer->emitStyleChanged();
+          emit rasterLayer->rendererChanged();
+        }
+      }
+    }
+  }
+}

src/app/qgisapp.h

@@ -1445,6 +1445,8 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
 
     void onSnappingConfigChanged();
 
+    void onCalculatedRenderDetailsChanged();
+
     /**
      * Triggers validation of the specified \a crs.
      */

src/core/raster/qgsrasterlayer.cpp

@@ -1613,9 +1613,6 @@ bool QgsRasterLayer::refreshRenderer( QgsRasterRenderer *rasterRenderer, const Q
         }
       }
 
-      emit repaintRequested();
-      emitStyleChanged();
-      emit rendererChanged();
       return true;
     }
 
@@ -1650,8 +1647,6 @@ bool QgsRasterLayer::refreshRenderer( QgsRasterRenderer *rasterRenderer, const Q
         }
       }
 
-      emitStyleChanged();
-      emit rendererChanged();
       refreshed = true;
     }
   }