FORCE users to make a choice in the transformation selection dialog

We don't want users to fallback to a "don't know what this means,
I'll click 'Cancel'" behaviour here. Doing so results in the
worst possible inaccurate transformations used. Literally, picking
ANYTHING in the dialog is a better selection then allowing
them to click cancel.

Sponsored by ICSM

src/app/qgsdatumtransformtablewidget.cpp

@@ -197,7 +197,7 @@ QgsDatumTransformTableWidget::QgsDatumTransformTableWidget( QWidget *parent )
 
 void QgsDatumTransformTableWidget::addDatumTransform()
 {
-  QgsDatumTransformDialog dlg( QgsCoordinateReferenceSystem(), QgsCoordinateReferenceSystem(), true, false );
+  QgsDatumTransformDialog dlg( QgsCoordinateReferenceSystem(), QgsCoordinateReferenceSystem(), true, false, false );
   if ( dlg.exec() )
   {
     QPair< QPair<QgsCoordinateReferenceSystem, int>, QPair<QgsCoordinateReferenceSystem, int > > dt = dlg.selectedDatumTransforms();
@@ -250,7 +250,7 @@ void QgsDatumTransformTableWidget::editDatumTransform()
     if ( sourceCrs.isValid() && destinationCrs.isValid() &&
          ( sourceTransform != -1 || destinationTransform != -1 ) )
     {
-      QgsDatumTransformDialog dlg( sourceCrs, destinationCrs, true, false, qMakePair( sourceTransform, destinationTransform ) );
+      QgsDatumTransformDialog dlg( sourceCrs, destinationCrs, true, false, false, qMakePair( sourceTransform, destinationTransform ) );
       if ( dlg.exec() )
       {
         QPair< QPair<QgsCoordinateReferenceSystem, int>, QPair<QgsCoordinateReferenceSystem, int > > dt = dlg.selectedDatumTransforms();

src/gui/qgsdatumtransformdialog.cpp

@@ -38,7 +38,7 @@ bool QgsDatumTransformDialog::run( const QgsCoordinateReferenceSystem &sourceCrs
     return true;
   }
 
-  QgsDatumTransformDialog dlg( sourceCrs, destinationCrs, false, true, qMakePair( -1, -1 ), parent );
+  QgsDatumTransformDialog dlg( sourceCrs, destinationCrs, false, true, true, qMakePair( -1, -1 ), parent );
   if ( dlg.shouldAskUserForSelection() )
   {
     if ( dlg.exec() )
@@ -62,7 +62,7 @@ bool QgsDatumTransformDialog::run( const QgsCoordinateReferenceSystem &sourceCrs
 }
 
 QgsDatumTransformDialog::QgsDatumTransformDialog( const QgsCoordinateReferenceSystem &sourceCrs,
-    const QgsCoordinateReferenceSystem &destinationCrs, const bool allowCrsChanges, const bool showMakeDefault,
+    const QgsCoordinateReferenceSystem &destinationCrs, const bool allowCrsChanges, const bool showMakeDefault, const bool forceChoice,
     QPair<int, int> selectedDatumTransforms,
     QWidget *parent,
     Qt::WindowFlags f )
@@ -76,6 +76,13 @@ QgsDatumTransformDialog::QgsDatumTransformDialog( const QgsCoordinateReferenceSy
   if ( !showMakeDefault )
     mMakeDefaultCheckBox->setVisible( false );
 
+  if ( forceChoice )
+  {
+    mButtonBox->removeButton( mButtonBox->button( QDialogButtonBox::Cancel ) );
+    setWindowFlags( windowFlags() | Qt::CustomizeWindowHint );
+    setWindowFlags( windowFlags() & ~Qt::WindowCloseButtonHint );
+  }
+
   mDatumTransformTableWidget->setColumnCount( 2 );
   QStringList headers;
   headers << tr( "Source Transform" ) << tr( "Destination Transform" ) ;
@@ -253,6 +260,14 @@ void QgsDatumTransformDialog::accept()
   QDialog::accept();
 }
 
+void QgsDatumTransformDialog::reject()
+{
+  if ( !mButtonBox->button( QDialogButtonBox::Cancel ) )
+    return; // users HAVE to make a choice, no click on the dialog "x" to avoid this!
+
+  QDialog::reject();
+}
+
 bool QgsDatumTransformDialog::shouldAskUserForSelection()
 {
   if ( mDatumTransforms.count() > 1 )

src/gui/qgsdatumtransformdialog.h

@@ -59,12 +59,14 @@ class GUI_EXPORT QgsDatumTransformDialog : public QDialog, private Ui::QgsDatumT
                              const QgsCoordinateReferenceSystem &destinationCrs = QgsCoordinateReferenceSystem(),
                              bool allowCrsChanges = false,
                              bool showMakeDefault = true,
+                             bool forceChoice = true,
                              QPair<int, int> selectedDatumTransforms = qMakePair( -1, -1 ),
                              QWidget *parent = nullptr,
                              Qt::WindowFlags f = nullptr );
     ~QgsDatumTransformDialog() override;
 
     void accept() override;
+    void reject() override;
 
     /**
      * Returns the source and destination transforms, each being a pair of QgsCoordinateReferenceSystems and datum transform code