[layouts] Fix incorrectly rendered item background and frame when item is

non-opaque

Fixes #20909

src/core/layout/qgslayoutitem.cpp

@@ -325,11 +325,16 @@ void QgsLayoutItem::paint( QPainter *painter, const QStyleOptionGraphicsItem *it
       // painter is already scaled to dots
       // need to translate so that item origin is at 0,0 in painter coordinates (not bounding rect origin)
       p.translate( -boundingRect().x() * context.scaleFactor(), -boundingRect().y() * context.scaleFactor() );
+      // scale to layout units for background and frame rendering
+      p.scale( context.scaleFactor(), context.scaleFactor() );
       drawBackground( context );
+      p.scale( 1 / context.scaleFactor(), 1 / context.scaleFactor() );
       double viewScale = QgsLayoutUtils::scaleFactorFromItemStyle( itemStyle );
       QgsLayoutItemRenderContext itemRenderContext( context, viewScale );
       draw( itemRenderContext );
+      p.scale( context.scaleFactor(), context.scaleFactor() );
       drawFrame( context );
+      p.scale( 1 / context.scaleFactor(), 1 / context.scaleFactor() );
       p.end();
 
       QgsImageOperation::multiplyOpacity( image, mEvaluatedOpacity );

tests/src/core/testqgslayoutitem.cpp

@@ -24,6 +24,7 @@
 #include "qgsreadwritecontext.h"
 #include "qgslayoutitemundocommand.h"
 #include "qgslayoutitemmap.h"
+#include "qgslayoutitemlabel.h"
 #include "qgslayoutitemshape.h"
 #include "qgslayouteffect.h"
 #include "qgsfillsymbollayer.h"
@@ -1979,6 +1980,21 @@ void TestQgsLayoutItem::opacity()
   checker.setControlPathPrefix( QStringLiteral( "composer_effects" ) );
   QVERIFY( checker.testLayout( mReport ) );
 
+  // with background and frame
+  l.removeLayoutItem( item );
+
+  QgsLayoutItemLabel *labelItem = new QgsLayoutItemLabel( &l );
+  l.addLayoutItem( labelItem );
+  labelItem->attemptSetSceneRect( QRectF( 50, 50, 150, 100 ) );
+  labelItem->setBackgroundEnabled( true );
+  labelItem->setBackgroundColor( QColor( 40, 140, 240 ) );
+  labelItem->setFrameEnabled( true );
+  labelItem->setFrameStrokeColor( QColor( 40, 30, 20 ) );
+  labelItem->setItemOpacity( 0.5 );
+  checker = QgsLayoutChecker( QStringLiteral( "composereffects_transparency_bgframe" ), &l );
+  checker.setControlPathPrefix( QStringLiteral( "composer_effects" ) );
+  QVERIFY( checker.testLayout( mReport ) );
+
   QgsLayout l2( QgsProject::instance() );
   l2.initializeDefaults();
   QgsLayoutItemShape *mComposerRect1 = new QgsLayoutItemShape( &l2 );