Add a render context flag so we know when a render operation is

occurring for drawing a symbol preview

Because these have no map association, certain settings like
sizes in "meters in map units" will have no meaning, and may need
special handling

Refs #28690

python/core/auto_generated/qgsrendercontext.sip.in

@@ -43,6 +43,7 @@ to be rendered etc.
       RenderPartialOutput,
       RenderPreviewJob,
       RenderBlocking,
+      RenderSymbolPreview,
     };
     typedef QFlags<QgsRenderContext::Flag> Flags;
 

src/core/layertree/qgslayertreemodel.cpp

@@ -1388,6 +1388,7 @@ QgsRenderContext *QgsLayerTreeModel::createTemporaryRenderContext() const
   context->setScaleFactor( dpi / 25.4 );
   context->setRendererScale( scale );
   context->setMapToPixel( QgsMapToPixel( mupp ) );
+  context->setFlag( QgsRenderContext::RenderSymbolPreview );
   return validData ? context.release() : nullptr;
 }
 

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -405,6 +405,7 @@ QgsRenderContext *QgsLayerTreeModelLegendNode::createTemporaryRenderContext() co
   context->setRendererScale( scale );
   context->setMapToPixel( QgsMapToPixel( mupp ) );
   context->setFlag( QgsRenderContext::Antialiasing, true );
+  context->setFlag( QgsRenderContext::RenderSymbolPreview, true );
   return context.release();
 }
 

src/core/qgsrendercontext.h

@@ -79,6 +79,7 @@ class CORE_EXPORT QgsRenderContext : public QgsTemporalRangeObject
       RenderPartialOutput      = 0x100, //!< Whether to make extra effort to update map image with partially rendered layers (better for interactive map canvas). Added in QGIS 3.0
       RenderPreviewJob         = 0x200, //!< Render is a 'canvas preview' render, and shortcuts should be taken to ensure fast rendering
       RenderBlocking           = 0x400, //!< Render and load remote sources in the same thread to ensure rendering remote sources (svg and images). WARNING: this flag must NEVER be used from GUI based applications (like the main QGIS application) or crashes will result. Only for use in external scripts or QGIS server.
+      RenderSymbolPreview      = 0x800, //!< The render is for a symbol preview only and map based properties may not be available, so care should be taken to handle map unit based sizes in an appropriate way.
     };
     Q_DECLARE_FLAGS( Flags, Flag )
 

src/core/symbology/qgssymbol.cpp

@@ -511,6 +511,7 @@ void QgsSymbol::drawPreviewIcon( QPainter *painter, QSize size, QgsRenderContext
   {
     tempContext.reset( new QgsRenderContext( QgsRenderContext::fromQPainter( painter ) ) );
     context = tempContext.get();
+    context->setFlag( QgsRenderContext::RenderSymbolPreview, true );
   }
 
   const bool prevForceVector = context->forceVectorOutput();
@@ -631,6 +632,7 @@ QImage QgsSymbol::bigSymbolPreviewImage( QgsExpressionContext *expressionContext
   }
 
   QgsRenderContext context = QgsRenderContext::fromQPainter( &p );
+  context.setFlag( QgsRenderContext::RenderSymbolPreview );
   if ( expressionContext )
     context.setExpressionContext( *expressionContext );
 

src/core/symbology/qgssymbollayerutils.cpp

@@ -838,6 +838,7 @@ QPicture QgsSymbolLayerUtils::symbolLayerPreviewPicture( const QgsSymbolLayer *l
   painter.setRenderHint( QPainter::Antialiasing );
   QgsRenderContext renderContext = QgsRenderContext::fromQPainter( &painter );
   renderContext.setForceVectorOutput( true );
+  renderContext.setFlag( QgsRenderContext::RenderSymbolPreview, true );
   QgsSymbolRenderContext symbolContext( renderContext, units, 1.0, false, nullptr, nullptr );
   std::unique_ptr< QgsSymbolLayer > layerClone( layer->clone() );
   layerClone->drawPreviewIcon( symbolContext, size );
@@ -853,6 +854,7 @@ QIcon QgsSymbolLayerUtils::symbolLayerPreviewIcon( const QgsSymbolLayer *layer,
   painter.begin( &pixmap );
   painter.setRenderHint( QPainter::Antialiasing );
   QgsRenderContext renderContext = QgsRenderContext::fromQPainter( &painter );
+  renderContext.setFlag( QgsRenderContext::RenderSymbolPreview );
   // build a minimal expression context
   QgsExpressionContext expContext;
   expContext.appendScopes( QgsExpressionContextUtils::globalProjectLayerScopes( nullptr ) );