Don't mark project dirty when legend nodes are expanded/collapsed

python/gui/layertree/qgslayertreeview.sip.in

@@ -148,6 +148,8 @@ Returns list of indicators associated with a particular layer tree node.
 .. versionadded:: 3.2
 %End
 
+
+
   public slots:
     void refreshLayerSymbology( const QString &layerId );
 %Docstring

src/app/qgisapp.cpp

@@ -3238,7 +3238,12 @@ void QgisApp::setupConnections()
   connect( mLayerTreeView->layerTreeModel()->rootGroup(), &QgsLayerTreeNode::visibilityChanged,
            this, &QgisApp::markDirty );
   connect( mLayerTreeView->layerTreeModel()->rootGroup(), &QgsLayerTreeNode::customPropertyChanged,
-           this, &QgisApp::markDirty );
+           this, [ = ]( QgsLayerTreeNode *, const QString & key )
+  {
+    // only mark dirty for non-view only changes
+    if ( !QgsLayerTreeView::viewOnlyCustomProperties().contains( key ) )
+      QgisApp::markDirty();
+  } );
 
   // connect map layer registry
   connect( QgsProject::instance(), &QgsProject::layersAdded,

src/gui/layertree/qgslayertreeview.cpp

@@ -355,6 +355,12 @@ QList<QgsLayerTreeViewIndicator *> QgsLayerTreeView::indicators( QgsLayerTreeNod
   return mIndicators.value( node );
 }
 
+///@cond PRIVATE
+QStringList QgsLayerTreeView::viewOnlyCustomProperties()
+{
+  return QStringList() << QStringLiteral( "expandedLegendNodes" );
+}
+///@endcond
 
 void QgsLayerTreeView::refreshLayerSymbology( const QString &layerId )
 {

src/gui/layertree/qgslayertreeview.h

@@ -135,6 +135,20 @@ class GUI_EXPORT QgsLayerTreeView : public QTreeView
      */
     QList<QgsLayerTreeViewIndicator *> indicators( QgsLayerTreeNode *node ) const;
 
+///@cond PRIVATE
+
+    /**
+     * Returns a list of custom property keys which are considered as related to view operations
+     * only. E.g. node expanded state.
+     *
+     * Changes to these keys will not mark a project as "dirty" and trigger unsaved changes
+     * warnings.
+     *
+     * \since QGIS 3.2
+     */
+    static QStringList viewOnlyCustomProperties() SIP_SKIP;
+///@endcond
+
   public slots:
     //! Force refresh of layer symbology. Normally not needed as the changes of layer's renderer are monitored by the model
     void refreshLayerSymbology( const QString &layerId );