Avoid use of composition map settings to set initial composer map extent

python/gui/qgscomposerview.sip

@@ -133,6 +133,9 @@ class QgsComposerView : QGraphicsView
      */
     void setPreviewMode( QgsPreviewEffect::PreviewMode mode );
 
+    void setMapCanvas( QgsMapCanvas* canvas );
+    QgsMapCanvas* mapCanvas() const;
+
   protected:
     void mousePressEvent( QMouseEvent* );
     void mouseReleaseEvent( QMouseEvent* );

src/app/composer/qgscomposer.cpp

@@ -4027,6 +4027,7 @@ void QgsComposer::createComposerView()
 
   delete mView;
   mView = new QgsComposerView();
+  mView->setMapCanvas( mQgis->mapCanvas() );
   mView->setContentsMargins( 0, 0, 0, 0 );
   mView->setHorizontalRuler( mHorizontalRuler );
   mView->setVerticalRuler( mVerticalRuler );

src/core/composer/qgscomposermap.cpp

@@ -85,9 +85,6 @@ QgsComposerMap::QgsComposerMap( QgsComposition *composition, int x, int y, int w
   int bgBlueInt = project->readNumEntry( QStringLiteral( "Gui" ), QStringLiteral( "/CanvasColorBluePart" ), 255 );
   setBackgroundColor( QColor( bgRedInt, bgGreenInt, bgBlueInt ) );
 
-  //calculate mExtent based on width/height ratio and map canvas extent
-  mExtent = mComposition->mapSettings().visibleExtent();
-
   init();
 
   setSceneRect( QRectF( x, y, width, height ) );
@@ -775,9 +772,14 @@ void QgsComposerMap::setNewExtent( const QgsRectangle& extent )
 void QgsComposerMap::zoomToExtent( const QgsRectangle &extent )
 {
   QgsRectangle newExtent = extent;
+  QgsRectangle currentExtent = *currentMapExtent();
   //Make sure the width/height ratio is the same as the current composer map extent.
   //This is to keep the map item frame size fixed
-  double currentWidthHeightRatio = currentMapExtent()->width() / currentMapExtent()->height();
+  double currentWidthHeightRatio = 1.0;
+  if ( !currentExtent.isNull() )
+    currentWidthHeightRatio = currentExtent.width() / currentExtent.height();
+  else
+    currentWidthHeightRatio = rect().width() / rect().height();
   double newWidthHeightRatio = newExtent.width() / newExtent.height();
 
   if ( currentWidthHeightRatio < newWidthHeightRatio )

src/gui/qgscomposerview.cpp

@@ -44,7 +44,7 @@
 #include "qgscomposerattributetablev2.h"
 #include "qgsaddremovemultiframecommand.h"
 #include "qgspaperitem.h"
-#include "qgsmapcanvas.h" //for QgsMapCanvas::WheelAction
+#include "qgsmapcanvas.h"
 #include "qgscursors.h"
 #include "qgscomposerutils.h"
 
@@ -997,6 +997,9 @@ void QgsComposerView::mouseReleaseEvent( QMouseEvent* e )
       else
       {
         QgsComposerMap* composerMap = new QgsComposerMap( composition(), mRubberBandItem->transform().dx(), mRubberBandItem->transform().dy(), mRubberBandItem->rect().width(), mRubberBandItem->rect().height() );
+        if ( mCanvas )
+          composerMap->zoomToExtent( mCanvas->mapSettings().visibleExtent() );
+
         composition()->addComposerMap( composerMap );
 
         composition()->setAllDeselected();
@@ -2103,6 +2106,16 @@ void QgsComposerView::setPreviewMode( QgsPreviewEffect::PreviewMode mode )
   mPreviewEffect->setMode( mode );
 }
 
+void QgsComposerView::setMapCanvas( QgsMapCanvas* canvas )
+{
+  mCanvas = canvas;
+}
+
+QgsMapCanvas*QgsComposerView::mapCanvas() const
+{
+  return mCanvas;
+}
+
 void QgsComposerView::paintEvent( QPaintEvent* event )
 {
   if ( mPaintingEnabled )

src/gui/qgscomposerview.h

@@ -39,6 +39,7 @@ class QgsComposerScaleBar;
 class QgsComposerShape;
 class QgsComposerNodesItem;
 class QgsComposerAttributeTableV2;
+class QgsMapCanvas;
 
 /** \ingroup gui
  * Widget to display the composer items. Manages the composer tools and the
@@ -170,6 +171,20 @@ class GUI_EXPORT QgsComposerView: public QGraphicsView
      */
     void setPreviewMode( QgsPreviewEffect::PreviewMode mode );
 
+    /** Sets the map canvas associated with the view. This allows the
+     * view to retrieve map settings from the canvas.
+     * @note added in QGIS 3.0
+     * @see mapCanvas()
+     */
+    void setMapCanvas( QgsMapCanvas* canvas );
+
+    /**
+     * Returns the map canvas associated with the view.
+     * @see setMapCanvas()
+     * @note added in QGIS 3.0
+     */
+    QgsMapCanvas* mapCanvas() const;
+
   protected:
     void mousePressEvent( QMouseEvent* ) override;
     void mouseReleaseEvent( QMouseEvent* ) override;
@@ -218,6 +233,8 @@ class GUI_EXPORT QgsComposerView: public QGraphicsView
     QgsComposerRuler* mHorizontalRuler;
     QgsComposerRuler* mVerticalRuler;
 
+    QgsMapCanvas* mCanvas = nullptr;
+
     //! Draw a shape on the canvas
     void addShape( Tool currentTool );