[layouts] Use polyline shape instead of bounding box for selection, makes polylines selectable via single click

Fixes #20940

Author: jdugge

python/core/auto_generated/layout/qgslayoutitempolyline.sip.in

@@ -53,6 +53,8 @@ The caller takes responsibility for deleting the returned object.
 
     virtual QString displayName() const;
 
+    virtual QPainterPath shape() const;
+
 
     QgsLineSymbol *symbol();
 %Docstring

src/core/layout/qgslayoutitempolyline.cpp

@@ -25,6 +25,7 @@
 #include "qgssvgcache.h"
 #include <QSvgRenderer>
 #include <limits>
+#include <QGraphicsPathItem>
 
 QgsLayoutItemPolyline::QgsLayoutItemPolyline( QgsLayout *layout )
   : QgsLayoutNodesItem( layout )
@@ -312,6 +313,19 @@ void QgsLayoutItemPolyline::setArrowHeadWidth( double width )
   update();
 }
 
+QPainterPath QgsLayoutItemPolyline::shape() const
+{
+  QPainterPath path;
+  path.addPolygon( mPolygon );
+
+  QPainterPathStroker ps;
+
+  ps.setWidth( 2 * mMaxSymbolBleed );
+  QPainterPath strokedOutline = ps.createStroke( path );
+
+  return strokedOutline;
+}
+
 void QgsLayoutItemPolyline::setStartSvgMarkerPath( const QString &path )
 {
   QSvgRenderer r;

src/core/layout/qgslayoutitempolyline.h

@@ -20,6 +20,9 @@
 #include "qgis_core.h"
 #include "qgslayoutitemnodeitem.h"
 #include "qgssymbol.h"
+#include <QGraphicsPathItem>
+#include "qgslogger.h"
+#include "qgslayout.h"
 
 /**
  * \ingroup core
@@ -61,6 +64,7 @@ class CORE_EXPORT QgsLayoutItemPolyline: public QgsLayoutNodesItem
     int type() const override;
     QIcon icon() const override;
     QString displayName() const override;
+    QPainterPath shape() const override;
 
     /**
      * Returns the line symbol used to draw the shape.