Respect scale lock when panning the map

python/gui/auto_generated/qgsmapcanvas.sip.in

@@ -1201,6 +1201,7 @@ called when panning is in action, reset indicates end of panning
 called on resize or changed extent to notify canvas items to change their rectangle
 %End
 
+      public:
 }; // class QgsMapCanvas
 
 

src/core/qgsmapsettings.cpp

@@ -226,6 +226,7 @@ void QgsMapSettings::updateDerived()
   QgsDebugMsgLevel( QStringLiteral( "Rotation: %1 degrees" ).arg( mRotation ), 5 );
   QgsDebugMsgLevel( QStringLiteral( "Extent: %1" ).arg( mExtent.asWktCoordinates() ), 5 );
   QgsDebugMsgLevel( QStringLiteral( "Visible Extent: %1" ).arg( mVisibleExtent.asWktCoordinates() ), 5 );
+  QgsDebugMsgLevel( QStringLiteral( "Magnification factor: %1" ).arg( mMagnificationFactor ), 5 );
 
   mValid = true;
 }

src/gui/qgsmapcanvas.cpp

@@ -1086,7 +1086,23 @@ void QgsMapCanvas::setExtent( const QgsRectangle &r, bool magnified )
   }
   else
   {
-    mSettings.setExtent( r, magnified );
+    // If scale is locked we need to maintain the current scale, so we
+    // - magnify and recenter the map
+    // - restore locked scale
+    if ( mScaleLocked && magnified )
+    {
+      ScaleRestorer restorer( this );
+      const double ratio { extent().width() / extent().height() };
+      const double factor { r.width() / r.height() > ratio ? extent().width() / r.width() :  extent().height() / r.height() };
+      const double scaleFactor { qBound( QgsGuiUtils::CANVAS_MAGNIFICATION_MIN,  mSettings.magnificationFactor() * factor, QgsGuiUtils::CANVAS_MAGNIFICATION_MAX ) };
+      const QgsPointXY newCenter { r.center() };
+      mSettings.setMagnificationFactor( scaleFactor, &newCenter );
+      emit magnificationChanged( scaleFactor );
+    }
+    else
+    {
+      mSettings.setExtent( r, magnified );
+    }
   }
   emit extentsChanged();
   updateScale();
@@ -1134,18 +1150,15 @@ bool QgsMapCanvas::setReferencedExtent( const QgsReferencedRectangle &extent )
 void QgsMapCanvas::setCenter( const QgsPointXY &center )
 {
   const QgsRectangle r = mapSettings().extent();
-  const double x = center.x();
-  const double y = center.y();
+  const double xMin = center.x() - r.width() / 2.0;
+  const double yMin = center.y() - r.height() / 2.0;
   const QgsRectangle rect(
-    x - r.width() / 2.0, y - r.height() / 2.0,
-    x + r.width() / 2.0, y + r.height() / 2.0
+    xMin, yMin,
+    xMin + r.width(), yMin + r.height()
   );
   if ( ! rect.isEmpty() )
   {
-    setExtent(
-      rect,
-      true
-    );
+    setExtent( rect, true );
   }
 } // setCenter
 
@@ -1929,6 +1942,7 @@ void QgsMapCanvas::zoomWithCenter( int x, int y, bool zoomIn )
 
   if ( mScaleLocked )
   {
+    ScaleRestorer restorer( this );
     setMagnificationFactor( mapSettings().magnificationFactor() / scaleFactor, &center );
   }
   else
@@ -2511,7 +2525,7 @@ void QgsMapCanvas::zoomByFactor( double scaleFactor, const QgsPointXY *center, b
 {
   if ( mScaleLocked && !ignoreScaleLock )
   {
-    // zoom map to mouse cursor by magnifying
+    ScaleRestorer restorer( this );
     setMagnificationFactor( mapSettings().magnificationFactor() / scaleFactor, center );
   }
   else

src/gui/qgsmapcanvas.h

@@ -1110,6 +1110,28 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
 
   private:
 
+    // Restore scale RAII
+    class ScaleRestorer
+    {
+      public:
+        ScaleRestorer( QgsMapCanvas *canvas ):
+          mCanvas( canvas )
+        {
+          mLockedScale = mCanvas->mapSettings().scale();
+        };
+
+        ~ScaleRestorer()
+        {
+          QgsRectangle newExtent = mCanvas->mapSettings().extent();
+          newExtent.scale( mLockedScale / mCanvas->mapSettings().scale() );
+          mCanvas->mSettings.setExtent( newExtent );
+        };
+
+      private:
+        QgsMapCanvas *mCanvas;
+        double mLockedScale;
+    };
+
     //! encompases all map settings necessary for map rendering
     QgsMapSettings mSettings;
 

tests/src/python/test_qgsmapcanvas.py

@@ -26,6 +26,7 @@
                        QgsPolygon,
                        QgsLineString,
                        QgsPoint,
+                       QgsPointXY,
                        QgsApplication)
 from qgis.gui import (QgsMapCanvas)
 
@@ -485,6 +486,85 @@ def testSaveMultipleCanvasesToProject(self):
         self.assertEqual(c4.expressionContextScope().variable('vara'), 2222)
         self.assertEqual(c4.expressionContextScope().variable('varc'), 'cc')
 
+    def testLockedScale(self):
+        """Test zoom/pan/center operations when scale lock is on"""
+
+        c = QgsMapCanvas()
+        dpr = c.mapSettings().devicePixelRatio()
+        self.assertEqual(c.size().width(), 640)
+        self.assertEqual(c.size().height(), 480)
+
+        c.setExtent(QgsRectangle(5, 45, 9, 47))
+        self.assertEqual(round(c.scale() / 100000), 13 * dpr)
+        c.zoomScale(2500000)
+        c.setScaleLocked(True)
+        self.assertEqual(round(c.magnificationFactor(), 1), 1)
+
+        # Test setExtent
+        c.setExtent(QgsRectangle(6, 45.5, 8, 46), True)
+        self.assertEqual(round(c.scale()), 2500000)
+        self.assertEqual(c.center().x(), 7.0)
+        self.assertEqual(c.center().y(), 45.75)
+        self.assertEqual(round(c.magnificationFactor()), 4 / dpr)
+
+        # Test setCenter
+        c.setCenter(QgsPointXY(6, 46))
+        self.assertEqual(c.center().x(), 6)
+        self.assertEqual(c.center().y(), 46)
+        self.assertEqual(round(c.scale()), 2500000)
+
+        # Test zoom
+        c.zoomByFactor(0.5, QgsPointXY(6.5, 46.5), False)
+        self.assertEqual(c.center().x(), 6.5)
+        self.assertEqual(c.center().y(), 46.5)
+        self.assertTrue(c.magnificationFactor() > 7 / dpr)
+        self.assertEqual(round(c.scale()), 2500000)
+
+        # Test zoom with center
+        # default zoom factor is 2, x and y are pixel coordinates, default size is 640x480
+        c.zoomWithCenter(300, 200, True)
+        self.assertEqual(round(c.center().x(), 1), 6.5)
+        self.assertEqual(round(c.center().y(), 1), 46.6)
+        self.assertEqual(round(c.scale()), 2500000)
+        self.assertTrue(c.magnificationFactor() > (14 / dpr) and c.magnificationFactor() < (16 / dpr))
+        # out ...
+        c.zoomWithCenter(300, 200, False)
+        self.assertEqual(round(c.center().x(), 1), 6.5)
+        self.assertEqual(round(c.center().y(), 1), 46.6)
+        self.assertEqual(round(c.scale()), 2500000)
+        self.assertTrue(c.magnificationFactor() > 7 / dpr)
+
+        # Test setExtent with different ratio
+        c2 = QgsMapCanvas()
+        c2.setExtent(QgsRectangle(5, 45, 9, 47))
+        c2.zoomScale(2500000)
+        c2.setScaleLocked(True)
+
+        c2.setExtent(QgsRectangle(3, 45, 11, 45.5), True)
+        self.assertEqual(round(c2.scale()), 2500000)
+        self.assertEqual(c2.center().x(), 7.0)
+        self.assertEqual(c2.center().y(), 45.25)
+        self.assertAlmostEqual(c2.magnificationFactor(), 1 / dpr, 0)
+
+        # Restore original
+        c2.setExtent(QgsRectangle(5, 45, 9, 47), True)
+        self.assertEqual(round(c2.scale()), 2500000)
+        self.assertEqual(c2.center().x(), 7.0)
+        self.assertEqual(c2.center().y(), 46.0)
+        self.assertAlmostEqual(c2.magnificationFactor(), 2 / dpr, 0)
+
+        c2.setExtent(QgsRectangle(7, 46, 11, 46.5), True)
+        self.assertEqual(round(c2.scale()), 2500000)
+        self.assertEqual(c2.center().x(), 9.0)
+        self.assertEqual(c2.center().y(), 46.25)
+        self.assertAlmostEqual(c2.magnificationFactor(), 2 / dpr, 0)
+
+        c2.setExtent(QgsRectangle(7, 46, 9, 46.5), True)
+        self.assertEqual(round(c2.scale()), 2500000)
+        self.assertEqual(c2.center().x(), 8.0)
+        self.assertEqual(c2.center().y(), 46.25)
+        self.assertAlmostEqual(c2.magnificationFactor(), 4 / dpr, 0)
+
 
 if __name__ == '__main__':
     unittest.main()