Allow creation of attribute form group boxes and rows which

aren't contained within tabs

This was always supported by the form, but blocked in the form
editor. It stems from a subtle twist in the original logic of
"tab containers can ONLY be top level items" which was misinterpreted
sometime as "top level items can ONLY be tabs".

Restore the original constraint that tabs can only be top
level items and allow also top level group boxes and rows

src/gui/qgsaddtaborgroup.cpp

@@ -38,7 +38,7 @@ QgsAddAttributeFormContainerDialog::QgsAddAttributeFormContainerDialog( QgsVecto
 
   mTypeCombo->setCurrentIndex( mTypeCombo->findData( QVariant::fromValue( Qgis::AttributeEditorContainerType::Tab ) ) );
 
-  mParentCombo->setEnabled( false );
+  mParentCombo->addItem( QString() );
   if ( !mExistingContainers.isEmpty() )
   {
     int i = 0;
@@ -53,12 +53,6 @@ QgsAddAttributeFormContainerDialog::QgsAddAttributeFormContainerDialog( QgsVecto
       ++i;
     }
   }
-  else
-  {
-    // top level items must be tabs
-    mTypeCombo->removeItem( mTypeCombo->findData( QVariant::fromValue( Qgis::AttributeEditorContainerType::GroupBox ) ) );
-    mTypeCombo->removeItem( mTypeCombo->findData( QVariant::fromValue( Qgis::AttributeEditorContainerType::Row ) ) );
-  }
 
   connect( buttonBox, &QDialogButtonBox::helpRequested, this, &QgsAddAttributeFormContainerDialog::showHelp );
 
@@ -80,6 +74,9 @@ QTreeWidgetItem *QgsAddAttributeFormContainerDialog::parentContainerItem()
   if ( containerType() == Qgis::AttributeEditorContainerType::Tab )
     return nullptr;
 
+  if ( !mParentCombo->currentData().isValid() )
+    return nullptr;
+
   const ContainerPair tab = mExistingContainers.at( mParentCombo->currentData().toInt() );
   return tab.second;
 }

src/gui/vector/qgsattributesformproperties.cpp

@@ -731,22 +731,11 @@ void QgsAttributesFormProperties::addContainer()
     return;
 
   const QString name = dialog.name();
-  switch ( dialog.containerType() )
-  {
-    case Qgis::AttributeEditorContainerType::Tab:
-    {
-      mFormLayoutTree->addContainer( mFormLayoutTree->invisibleRootItem(), name, dialog.columnCount(), Qgis::AttributeEditorContainerType::Tab );
-      break;
-    }
-
-    case Qgis::AttributeEditorContainerType::GroupBox:
-    case Qgis::AttributeEditorContainerType::Row:
-    {
-      QTreeWidgetItem *parentContainerItem = dialog.parentContainerItem();
-      mFormLayoutTree->addContainer( parentContainerItem, name, dialog.columnCount(), dialog.containerType() );
-      break;
-    }
-  }
+  QTreeWidgetItem *parentContainerItem = dialog.parentContainerItem();
+  mFormLayoutTree->addContainer( parentContainerItem ? parentContainerItem : mFormLayoutTree->invisibleRootItem(),
+                                 name,
+                                 dialog.columnCount(),
+                                 dialog.containerType() );
 }
 
 void QgsAttributesFormProperties::removeTabOrGroupButton()
@@ -795,7 +784,14 @@ QgsAttributeEditorElement *QgsAttributesFormProperties::createAttributeEditorWid
     {
       QgsAttributeEditorContainer *container = new QgsAttributeEditorContainer( item->text( 0 ), parent, itemData.backgroundColor() );
       container->setColumnCount( itemData.columnCount() );
-      container->setType( isTopLevel ? Qgis::AttributeEditorContainerType::Tab : itemData.containerType() );
+      // only top-level containers can be tabs
+      Qgis::AttributeEditorContainerType type = itemData.containerType();
+      if ( type == Qgis::AttributeEditorContainerType::Tab && !isTopLevel )
+      {
+        // a top container found which isn't at the top level -- reset it to a group box instead
+        type = Qgis::AttributeEditorContainerType::GroupBox;
+      }
+      container->setType( type );
       container->setCollapsed( itemData.collapsed() );
       container->setCollapsedExpression( itemData.collapsedExpression() );
       container->setVisibilityExpression( itemData.visibilityExpression() );