[Backport release-3_34] Fix crash on drawing curved text with zero width characters

src/core/textrenderer/qgstextrenderer.cpp

@@ -324,7 +324,11 @@ void QgsTextRenderer::drawDocumentOnLine( const QPolygonF &line, const QgsTextFo
   if ( placement->graphemePlacement.empty() )
     return;
 
-  std::vector< QgsTextRenderer::Component > components;
+  // We may have deliberately skipped over some graphemes during curved text placement (such as zero-width graphemes).
+  // So we need to use a hash of the original grapheme index to place generated components in, as there may accordingly
+  // be graphemes which don't result in components, and we can't just blindly assume the component array position
+  // will match the original grapheme index
+  QHash< int, QgsTextRenderer::Component > components;
   components.reserve( placement->graphemePlacement.size() );
   for ( const QgsTextRendererUtils::CurvedGraphemePlacement &grapheme : std::as_const( placement->graphemePlacement ) )
   {
@@ -340,7 +344,7 @@ void QgsTextRenderer::drawDocumentOnLine( const QPolygonF &line, const QgsTextFo
       component.origin.ry() += verticalOffset * std::sin( grapheme.angle + M_PI_2 );
     }
 
-    components.emplace_back( component );
+    components.insert( grapheme.graphemeIndex, component );
   }
 
   if ( format.background().enabled() )

tests/src/python/test_qgstextrenderer.py

@@ -3842,6 +3842,42 @@ def testDrawTextOnLineAtStart(self):
         painter.end()
         self.assertTrue(self.image_check('text_on_line_at_start', 'text_on_line_at_start', image, 'text_on_line_at_start'))
 
+    def testDrawTextOnLineZeroWidthChar(self):
+        format = QgsTextFormat()
+        format.setFont(getTestFont('bold'))
+        format.setSize(16)
+        format.setSizeUnit(QgsUnitTypes.RenderUnit.RenderPoints)
+        format.background().setEnabled(True)
+
+        image = QImage(400, 400, QImage.Format.Format_RGB32)
+
+        painter = QPainter()
+        ms = QgsMapSettings()
+        ms.setExtent(QgsRectangle(0, 0, 50, 50))
+        ms.setOutputSize(image.size())
+        context = QgsRenderContext.fromMapSettings(ms)
+        context.setPainter(painter)
+        context.setScaleFactor(96 / 25.4)  # 96 DPI
+        context.setFlag(QgsRenderContext.Flag.ApplyScalingWorkaroundForTextRendering, True)
+
+        painter.begin(image)
+        painter.setRenderHint(QPainter.RenderHint.Antialiasing)
+        image.fill(QColor(152, 219, 249))
+
+        painter.setBrush(Qt.BrushStyle.NoBrush)
+        painter.setPen(QPen(QColor(0, 0, 0)))
+
+        line = QPolygonF([QPointF(50, 200), QPointF(350, 200)])
+        painter.drawPolygon(line)
+
+        painter.setBrush(QBrush(QColor(182, 239, 255)))
+        painter.setPen(Qt.PenStyle.NoPen)
+
+        QgsTextRenderer.drawTextOnLine(line, 'b\r\na', context, format, 0)
+
+        painter.end()
+        self.assertTrue(self.image_check('text_on_curved_line_zero_width_char', 'text_on_curved_line_zero_width_char', image, 'text_on_curved_line_zero_width_char'))
+
     def testDrawTextOnLineAtOffset(self):
         format = QgsTextFormat()
         format.setFont(getTestFont('bold'))