Restore attribute table column sizes in the Merge Attributes dialog

Otherwise the columns all get set to a minimal default width,
which quickly gets very frustrating if you're merging a lot of
features and have to keep manually resizing columns to see cell content...

src/app/qgsmergeattributesdialog.cpp

@@ -110,6 +110,8 @@ QgsMergeAttributesDialog::QgsMergeAttributesDialog( const QgsFeatureList &featur
 
   connect( mSkipAllButton, &QAbstractButton::clicked, this, &QgsMergeAttributesDialog::setAllToSkip );
   connect( mTableWidget, &QTableWidget::cellChanged, this, &QgsMergeAttributesDialog::tableWidgetCellChanged );
+
+  setAttributeTableConfig( mVectorLayer->attributeTableConfig() );
 }
 
 QgsMergeAttributesDialog::QgsMergeAttributesDialog()
@@ -126,6 +128,29 @@ QgsMergeAttributesDialog::~QgsMergeAttributesDialog()
   delete mSelectionRubberBand;
 }
 
+void QgsMergeAttributesDialog::setAttributeTableConfig( const QgsAttributeTableConfig &config )
+{
+  const QVector< QgsAttributeTableConfig::ColumnConfig > columns = config.columns();
+  for ( const QgsAttributeTableConfig::ColumnConfig &columnConfig : columns )
+  {
+    if ( columnConfig.hidden )
+      continue;
+
+    const int col = mFieldToColumnMap.value( columnConfig.name, -1 );
+    if ( col < 0 )
+      continue;
+
+    if ( columnConfig.width >= 0 )
+    {
+      mTableWidget->setColumnWidth( col, columnConfig.width );
+    }
+    else
+    {
+      mTableWidget->setColumnWidth( col, mTableWidget->horizontalHeader()->defaultSectionSize() );
+    }
+  }
+}
+
 void QgsMergeAttributesDialog::createTableWidgetContents()
 {
   //get information about attributes from vector layer
@@ -152,6 +177,7 @@ void QgsMergeAttributesDialog::createTableWidgetContents()
     }
 
     mTableWidget->setColumnCount( col + 1 );
+    mFieldToColumnMap[ mFields.at( idx ).name() ] = col;
 
     QComboBox *cb = createMergeComboBox( mFields.at( idx ).type() );
     if ( mFields.at( idx ).constraints().constraints() & QgsFieldConstraints::ConstraintUnique )

src/app/qgsmergeattributesdialog.h

@@ -29,6 +29,7 @@ class QgsMapCanvas;
 class QgsRubberBand;
 class QgsVectorLayer;
 class QComboBox;
+class QgsAttributeTableConfig;
 
 
 //! A dialog to insert the merge behavior for attributes (e.g. for the union features editing tool)
@@ -72,6 +73,8 @@ class APP_EXPORT QgsMergeAttributesDialog: public QDialog, private Ui::QgsMergeA
   private:
     QgsMergeAttributesDialog(); //default constructor forbidden
     void createTableWidgetContents();
+    void setAttributeTableConfig( const QgsAttributeTableConfig &config );
+
     //! Create new combo box with the options for featureXX / mean / min / max
     QComboBox *createMergeComboBox( QVariant::Type columnType ) const;
 
@@ -106,6 +109,7 @@ class APP_EXPORT QgsMergeAttributesDialog: public QDialog, private Ui::QgsMergeA
 
     QgsFields mFields;
     QSet<int> mHiddenAttributes;
+    QMap< QString, int > mFieldToColumnMap;
     bool mUpdating = false;
 
     static const QList< QgsStatisticalSummary::Statistic > DISPLAY_STATS;