[layouts] Fix polyline does not render purely horizontal/vertical lines

src/core/layout/qgslayoutitemnodeitem.cpp

@@ -295,8 +295,10 @@ void QgsLayoutNodesItem::rescaleToFitBoundingBox()
   const QRectF boundingRect = mPolygon.boundingRect();
 
   // compute x/y ratio
-  const float ratioX = rect().width() / boundingRect.width();
-  const float ratioY = rect().height() / boundingRect.height();
+  const float ratioX = !qgsDoubleNear( boundingRect.width(), 0.0 )
+                       ? rect().width() / boundingRect.width() : 0;
+  const float ratioY = !qgsDoubleNear( boundingRect.height(), 0.0 )
+                       ? rect().height() / boundingRect.height() : 0;
 
   // scaling
   QTransform trans;

tests/src/python/test_qgslayoutpolyline.py

@@ -306,6 +306,56 @@ class TestQgsLayoutPolyline(unittest.TestCase):
         self.assertEqual(bounds.top(), -3.0)
         self.assertEqual(bounds.bottom(), 93.0)
 
+    def testHorizontalLine(self):
+        pr = QgsProject()
+        l = QgsLayout(pr)
+        l.initializeDefaults()
+
+        p = QPolygonF()
+        p.append(QPointF(50.0, 100.0))
+        p.append(QPointF(100.0, 100.0))
+        shape = QgsLayoutItemPolyline(p, l)
+        l.addLayoutItem(shape)
+
+        props = {}
+        props["color"] = "0,0,0,255"
+        props["width"] = "10.0"
+        props["capstyle"] = "square"
+
+        style = QgsLineSymbol.createSimple(props)
+        shape.setSymbol(style)
+
+        checker = QgsLayoutChecker(
+            'composerpolyline_hozline', l)
+        checker.setControlPathPrefix("composer_polyline")
+        myTestResult, myMessage = checker.testLayout()
+        assert myTestResult, myMessage
+
+    def testVerticalLine(self):
+        pr = QgsProject()
+        l = QgsLayout(pr)
+        l.initializeDefaults()
+
+        p = QPolygonF()
+        p.append(QPointF(100.0, 50.0))
+        p.append(QPointF(100.0, 100.0))
+        shape = QgsLayoutItemPolyline(p, l)
+        l.addLayoutItem(shape)
+
+        props = {}
+        props["color"] = "0,0,0,255"
+        props["width"] = "10.0"
+        props["capstyle"] = "square"
+
+        style = QgsLineSymbol.createSimple(props)
+        shape.setSymbol(style)
+
+        checker = QgsLayoutChecker(
+            'composerpolyline_vertline', l)
+        checker.setControlPathPrefix("composer_polyline")
+        myTestResult, myMessage = checker.testLayout()
+        assert myTestResult, myMessage
+
 
 if __name__ == '__main__':
     unittest.main()