Fix undersized layertree icons on hidpi displays

(cherry picked from commit 27bb2c1)

Author: nyalldawson

python/core/auto_generated/layertree/qgslayertreemodel.sip.in

@@ -307,6 +307,17 @@ Gets map of map layer style overrides (key: layer ID, value: style name) where a
 Set map of map layer style overrides (key: layer ID, value: style name) where a different style should be used instead of the current one
 
 .. versionadded:: 2.10
+%End
+
+    static int scaleIconSize( int standardSize );
+%Docstring
+Scales an layer tree model icon size to compensate for display pixel density, making the icon
+size hi-dpi friendly, whilst still resulting in pixel-perfect sizes for low-dpi
+displays.
+
+``standardSize`` should be set to a standard icon size, e.g. 16, 24, 48, etc.
+
+.. versionadded:: 3.6
 %End
 
   protected slots:

src/core/layertree/qgslayertreemodel.cpp

@@ -227,10 +227,11 @@ QVariant QgsLayerTreeModel::data( const QModelIndex &index, int role ) const
 
       if ( vlayer && vlayer->isEditable() )
       {
-        QPixmap pixmap( icon.pixmap( 16, 16 ) );
+        const int iconSize = scaleIconSize( 16 );
+        QPixmap pixmap( icon.pixmap( iconSize, iconSize ) );
 
         QPainter painter( &pixmap );
-        painter.drawPixmap( 0, 0, 16, 16, QgsApplication::getThemePixmap( vlayer->isModified() ? "/mIconEditableEdits.svg" : "/mActionToggleEditing.svg" ) );
+        painter.drawPixmap( 0, 0, iconSize, iconSize, QgsApplication::getThemePixmap( vlayer->isModified() ? QStringLiteral( "/mIconEditableEdits.svg" ) : QStringLiteral( "/mActionToggleEditing.svg" ) ) );
         painter.end();
 
         icon = QIcon( pixmap );
@@ -691,6 +692,13 @@ void QgsLayerTreeModel::setLayerStyleOverrides( const QMap<QString, QString> &ov
   mLayerStyleOverrides = overrides;
 }
 
+int QgsLayerTreeModel::scaleIconSize( int standardSize )
+{
+  QFontMetrics fm( ( QFont() ) );
+  const double scale = 1.1 * standardSize / 24;
+  return static_cast< int >( std::floor( std::max( Qgis::UI_SCALE_FACTOR * fm.height() * scale, static_cast< double >( standardSize ) ) ) );
+}
+
 void QgsLayerTreeModel::nodeWillAddChildren( QgsLayerTreeNode *node, int indexFrom, int indexTo )
 {
   Q_ASSERT( node );

src/core/layertree/qgslayertreemodel.h

@@ -274,6 +274,17 @@ class CORE_EXPORT QgsLayerTreeModel : public QAbstractItemModel
      */
     void setLayerStyleOverrides( const QMap<QString, QString> &overrides );
 
+    /**
+     * Scales an layer tree model icon size to compensate for display pixel density, making the icon
+     * size hi-dpi friendly, whilst still resulting in pixel-perfect sizes for low-dpi
+     * displays.
+     *
+     * \a standardSize should be set to a standard icon size, e.g. 16, 24, 48, etc.
+     *
+     * \since QGIS 3.6
+     */
+    static int scaleIconSize( int standardSize );
+
   protected slots:
     void nodeWillAddChildren( QgsLayerTreeNode *node, int indexFrom, int indexTo );
     void nodeAddedChildren( QgsLayerTreeNode *node, int indexFrom, int indexTo );

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -128,13 +128,14 @@ QSizeF QgsLayerTreeModelLegendNode::drawSymbolText( const QgsLegendSettings &set
 
 // -------------------------------------------------------------------------
 
-
 QgsSymbolLegendNode::QgsSymbolLegendNode( QgsLayerTreeLayer *nodeLayer, const QgsLegendSymbolItem &item, QObject *parent )
   : QgsLayerTreeModelLegendNode( nodeLayer, parent )
   , mItem( item )
   , mSymbolUsesMapUnits( false )
-  , mIconSize( 16, 16 )
 {
+  const int iconSize = QgsLayerTreeModel::scaleIconSize( 16 );
+  mIconSize = QSize( iconSize, iconSize );
+
   updateLabel();
   connect( qobject_cast<QgsVectorLayer *>( nodeLayer->layer() ), &QgsVectorLayer::symbolFeatureCountMapChanged, this, &QgsSymbolLegendNode::updateLabel );
   connect( nodeLayer, &QObject::destroyed, this, [ = ]() { mLayerNode = nullptr; } );
@@ -160,19 +161,21 @@ QSize QgsSymbolLegendNode::minimumIconSize() const
 
 QSize QgsSymbolLegendNode::minimumIconSize( QgsRenderContext *context ) const
 {
-  QSize minSz( 16, 16 );
+  const int iconSize = QgsLayerTreeModel::scaleIconSize( 16 );
+  const int largeIconSize = QgsLayerTreeModel::scaleIconSize( 512 );
+  QSize minSz( iconSize, iconSize );
   if ( mItem.symbol() && mItem.symbol()->type() == QgsSymbol::Marker )
   {
     minSz = QgsImageOperation::nonTransparentImageRect(
-              QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( 512, 512 ), 0,
+              QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( largeIconSize, largeIconSize ), 0,
                   context ).toImage(),
               minSz,
               true ).size();
   }
   else if ( mItem.symbol() && mItem.symbol()->type() == QgsSymbol::Line )
   {
     minSz = QgsImageOperation::nonTransparentImageRect(
-              QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( minSz.width(), 512 ), 0,
+              QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( minSz.width(), largeIconSize ), 0,
                   context ).toImage(),
               minSz,
               true ).size();
@@ -576,8 +579,8 @@ QVariant QgsRasterSymbolLegendNode::data( int role ) const
 {
   if ( role == Qt::DecorationRole )
   {
-    QSize iconSize( 16, 16 ); // TODO: configurable?
-    QPixmap pix( iconSize );
+    const int iconSize = QgsLayerTreeModel::scaleIconSize( 16 ); // TODO: configurable?
+    QPixmap pix( iconSize, iconSize );
     pix.fill( mColor );
     return QIcon( pix );
   }