Fix failing test

Author: nyalldawson

python/core/qgstextrenderer.sip.in

@@ -1629,7 +1629,7 @@ formats like SVG to maintain text as text objects, but at the cost of degraded
 rendering and may result in side effects like misaligned text buffers.
 %End
 
-    static QFontMetricsF fontMetrics( const QgsRenderContext &context, const QgsTextFormat &format );
+    static QFontMetricsF fontMetrics( QgsRenderContext &context, const QgsTextFormat &format );
 %Docstring
 Returns the font metrics for the given text ``format``, when rendered
 in the specified render ``context``. The font metrics will take into account

src/core/qgstextrenderer.cpp

@@ -1873,9 +1873,9 @@ void QgsTextRenderer::drawPart( QPointF origin, double rotation, QgsTextRenderer
   }
 }
 
-QFontMetricsF QgsTextRenderer::fontMetrics( const QgsRenderContext &context, const QgsTextFormat &format )
+QFontMetricsF QgsTextRenderer::fontMetrics( QgsRenderContext &context, const QgsTextFormat &format )
 {
-  return QFontMetricsF( format.scaledFont( context ) );
+  return QFontMetricsF( format.scaledFont( context ), context.painter() ? context.painter()->device() : nullptr );
 }
 
 void QgsTextRenderer::drawBuffer( QgsRenderContext &context, const QgsTextRenderer::Component &component, const QgsTextFormat &format )

src/core/qgstextrenderer.h

@@ -1408,7 +1408,7 @@ class CORE_EXPORT QgsTextRenderer
      * all scaling required by the render context.
      * \since QGIS 3.2
      */
-    static QFontMetricsF fontMetrics( const QgsRenderContext &context, const QgsTextFormat &format );
+    static QFontMetricsF fontMetrics( QgsRenderContext &context, const QgsTextFormat &format );
 
     /**
      * Returns the width of a text based on a given format.

tests/src/python/test_qgstextrenderer.py

@@ -448,18 +448,17 @@ def testFontMetrics(self):
 
         string = 'xxxxxxxxxxxxxxxxxxxxxx'
 
-        # calculated expected width
-        f = s.toQFont()
-        expected = QFontMetricsF(f).width(string)
-        scale = expected / 416.625
-
-        context = QgsRenderContext()
+        image = QImage(400, 400, QImage.Format_RGB32)
+        painter = QPainter(image)
+        context = QgsRenderContext.fromQPainter(painter)
         context.setScaleFactor(1)
         metrics = QgsTextRenderer.fontMetrics(context, s)
-        self.assertAlmostEqual(metrics.width(string), 51.9 * scale, -1)
         context.setScaleFactor(2)
-        metrics = QgsTextRenderer.fontMetrics(context, s)
-        self.assertAlmostEqual(metrics.width(string), 104.15 * scale, -1)
+        metrics2 = QgsTextRenderer.fontMetrics(context, s)
+        painter.end()
+
+        self.assertAlmostEqual(metrics.width(string), 51.9, 1)
+        self.assertAlmostEqual(metrics2.width(string), 104.15, 1)
 
     def imageCheck(self, name, reference_image, image):
         self.report += "<h2>Render {}</h2>\n".format(name)