[layouts] Fix line height setting not respected for HTML-enabled label items

python/core/auto_generated/textrenderer/qgstextformat.sip.in

@@ -729,6 +729,20 @@ Returns a pixmap preview for a text ``format``.
 :param screen: can be used to specify the destination screen properties for the icon. This allows the icon to be generated using the correct DPI and device pixel ratio for the target screen (since QGIS 3.32)
 
 .. versionadded:: 3.10
+%End
+
+    QString asCSS( double pointToPixelMultiplier = 1.0 ) const;
+%Docstring
+Returns a CSS string representing the specified text format as closely as possible.
+
+:param pointToPixelMultiplier: scaling factor to apply to convert point sizes to pixel font sizes.
+                               The CSS returned by this function will always use pixels for font sizes, so this parameter
+                               should be set to a suitable value to convert point sizes to pixels (e.g., taking into account
+                               destination DPI)
+
+:return: partial CSS string, e.g., "line-height: 120%;"
+
+.. versionadded:: 3.34
 %End
 
 };

src/core/layout/qgslayout.cpp

@@ -1124,6 +1124,12 @@ QList< QgsLayoutItem * > QgsLayout::addItemsFromXml( const QDomElement &parentEl
       {
         if ( label->mode() == QgsLayoutItemLabel::ModeHtml )
         {
+          QgsTextFormat textFormat = label->textFormat();
+          if ( textFormat.lineHeightUnit() == Qgis::RenderUnit::Percentage )
+          {
+            textFormat.setLineHeight( textFormat.lineHeight() + 0.22 );
+            label->setTextFormat( textFormat );
+          }
           QgsLayoutMultiFrame *html = QgsLayoutItemHtml::createFromLabel( label );
           addMultiFrame( html );
           if ( item->isGroupMember() )

src/core/layout/qgslayoutitemlabel.cpp

@@ -641,12 +641,9 @@ double QgsLayoutItemLabel::htmlUnitsToLayoutUnits()
 QString QgsLayoutItemLabel::createStylesheet() const
 {
   QString stylesheet;
+
   stylesheet += QStringLiteral( "body { margin: %1 %2;" ).arg( std::max( mMarginY * mHtmlUnitsToLayoutUnits, 0.0 ) ).arg( std::max( mMarginX * mHtmlUnitsToLayoutUnits, 0.0 ) );
-
-  QFont f = createDefaultFont();
-  stylesheet += QgsFontUtils::asCSS( f, 0.352778 * mHtmlUnitsToLayoutUnits );
-
-  stylesheet += QStringLiteral( "color: rgba(%1,%2,%3,%4);" ).arg( mFormat.color().red() ).arg( mFormat.color().green() ).arg( mFormat.color().blue() ).arg( QString::number( mFormat.color().alphaF(), 'f', 4 ) );
+  stylesheet += mFormat.asCSS( 0.352778 * mHtmlUnitsToLayoutUnits );
   stylesheet += QStringLiteral( "text-align: %1; }" ).arg( mHAlignment == Qt::AlignLeft ? QStringLiteral( "left" ) : mHAlignment == Qt::AlignRight ? QStringLiteral( "right" ) : mHAlignment == Qt::AlignHCenter ? QStringLiteral( "center" ) : QStringLiteral( "justify" ) );
 
   return stylesheet;

src/core/textrenderer/qgstextformat.cpp

@@ -1225,3 +1225,40 @@ QPixmap QgsTextFormat::textFormatPreviewPixmap( const QgsTextFormat &format, QSi
   painter.end();
   return pixmap;
 }
+
+QString QgsTextFormat::asCSS( double pointToPixelMultiplier ) const
+{
+  QString css;
+
+  switch ( lineHeightUnit() )
+  {
+    case Qgis::RenderUnit::Percentage:
+      css += QStringLiteral( "line-height: %1%;" ).arg( lineHeight() * 100 );
+      break;
+    case Qgis::RenderUnit::Pixels:
+      css += QStringLiteral( "line-height: %1px;" ).arg( lineHeight() );
+      break;
+    case Qgis::RenderUnit::Points:
+      // While the Qt documentation states pt unit type is supported, it's ignored, convert to px
+      css += QStringLiteral( "line-height: %1px;" ).arg( lineHeight() * pointToPixelMultiplier );
+      break;
+    case Qgis::RenderUnit::Millimeters:
+      // While the Qt documentation states cm unit type is supported, it's ignored, convert to px
+      css += QStringLiteral( "line-height: %1px;" ).arg( lineHeight() * 2.83464567 * pointToPixelMultiplier );
+      break;
+    case Qgis::RenderUnit::MetersInMapUnits:
+    case Qgis::RenderUnit::MapUnits:
+    case Qgis::RenderUnit::Inches:
+    case Qgis::RenderUnit::Unknown:
+      break;
+  }
+  css += QStringLiteral( "color: rgba(%1,%2,%3,%4);" ).arg( color().red() ).arg( color().green() ).arg( color().blue() ).arg( QString::number( color().alphaF(), 'f', 4 ) );
+  QFont f = toQFont();
+  if ( sizeUnit() == Qgis::RenderUnit::Millimeters )
+  {
+    f.setPointSizeF( size() / 0.352778 );
+  }
+  css += QgsFontUtils::asCSS( toQFont(), pointToPixelMultiplier );
+
+  return css;
+}

src/core/textrenderer/qgstextformat.h

@@ -657,6 +657,17 @@ class CORE_EXPORT QgsTextFormat
     */
     static QPixmap textFormatPreviewPixmap( const QgsTextFormat &format, QSize size, const QString &previewText = QString(), int padding = 0, const QgsScreenProperties &screen = QgsScreenProperties() );
 
+    /**
+     * Returns a CSS string representing the specified text format as closely as possible.
+     * \param pointToPixelMultiplier scaling factor to apply to convert point sizes to pixel font sizes.
+     * The CSS returned by this function will always use pixels for font sizes, so this parameter
+     * should be set to a suitable value to convert point sizes to pixels (e.g., taking into account
+     * destination DPI)
+     * \returns partial CSS string, e.g., "line-height: 120%;"
+     * \since QGIS 3.34
+     */
+    QString asCSS( double pointToPixelMultiplier = 1.0 ) const;
+
   private:
 
     QgsTextBufferSettings mBufferSettings;

tests/src/core/testqgslayoutlabel.cpp

@@ -55,6 +55,7 @@ class TestQgsLayoutLabel : public QgsTest
     void marginMethods(); //tests getting/setting margins
     void render();
     void renderAsHtml();
+    void renderAsHtmlLineHeight();
 #ifdef WITH_QTWEBKIT
     void convertToHtml();
     void renderAsHtmlRelative();
@@ -313,6 +314,34 @@ void TestQgsLayoutLabel::renderAsHtml()
   QVERIFY( layoutCheck( QStringLiteral( "composerlabel_renderhtml" ), &l, 0, 10 ) );
 }
 
+void TestQgsLayoutLabel::renderAsHtmlLineHeight()
+{
+  QgsLayout l( QgsProject::instance() );
+  l.initializeDefaults();
+
+  QgsLayoutItemLabel *label = new QgsLayoutItemLabel( &l );
+  label->setMargin( 1 );
+  l.addLayoutItem( label );
+
+  label->setText( QStringLiteral( "test <i>html</i><br>with <u>line height</u>." ) );
+
+  QgsTextFormat format;
+  format.setFont( QgsFontUtils::getStandardTestFont( QStringLiteral( "Bold" ) ) );
+  format.setSize( 48 );
+  format.setSizeUnit( Qgis::RenderUnit::Points );
+  format.setColor( QColor( 200, 40, 60 ) );
+  format.setLineHeight( 2.0 );
+  format.setLineHeightUnit( Qgis::RenderUnit::Percentage );
+  label->setTextFormat( format );
+
+  label->setPos( 70, 70 );
+  label->adjustSizeToText();
+  label->setMode( QgsLayoutItemLabel::ModeHtml );
+  label->update();
+
+  QVERIFY( layoutCheck( QStringLiteral( "composerlabel_renderhtmllineheight" ), &l, 0, 10 ) );
+}
+
 #ifdef WITH_QTWEBKIT
 void TestQgsLayoutLabel::convertToHtml()
 {