feat(alter-page): INSERT INTO container — append widgets as a container's children

ALTER PAGE could only INSERT BEFORE/AFTER a sibling widget, so there was no way to
fill an EMPTY container or add a widget to a container/dataview without an existing
sibling to anchor to. Add:

  alter page Mod.Page { insert into containerName { <widgets> } }

which appends the widgets as the last children of the named container. Full-stack:
grammar (INSERT INTO), visitor (Position "INTO"), executor (children take the
target's own entity context, e.g. a dataview's entity), and both mutators
(pagemutator for MPR files, MCP for live Studio Pro). An empty container omits its
Widgets list, so it is created with the Mendix widget-list marker and written back
into the parent (the bson.D grows). Simple containers are supported (DivContainer,
Container, DataView, GroupBox, ScrollContainerRegion, Section); LayoutGrid and
TabContainer have no single child list and give a clear error pointing to
INSERT BEFORE/AFTER inside the target column/tab.

Verified on 11.12.1: insert into an empty container, append to a non-empty one, and
insert a data-bound widget into a dataview — mx check 0 errors. Unit test for the
mutator INTO path + error case; doctype example; skill / quick-reference / syntax
help updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JXnEgoc2NQP1Y2TWMCMXC4

Author: claude

.claude/skills/mendix/alter-page.md

@@ -134,10 +134,22 @@ insert after txtName {
 insert before btnSave {
   actionbutton btnPreview (caption: 'Preview', action: microflow Module.ACT_Preview)
 }
+
+-- Insert INTO a container — append as its last child (works on an EMPTY container)
+insert into ctnToolbar {
+  actionbutton btnNew (caption: 'New', action: nothing, buttonstyle: primary)
+}
 ```
 
 Inserted widgets use the same syntax as `create page`. Multiple widgets can be inserted in a single block.
 
+`insert into <container>` appends as the last child of the named container — the
+only way to fill an **empty** container, and handy for adding to a container/dataview
+without needing a sibling to anchor to. Widgets inserted into a dataview take that
+dataview's entity as their context. Supported on simple containers (container,
+dataview, groupbox, scroll-container region); for a layout grid or tab container,
+insert relative to a widget inside the target column/tab instead.
+
 ### DROP - Remove Widgets
 
 ```sql

cmd/mxcli/syntax/features_page.go

@@ -89,7 +89,7 @@ func init() {
 			"set property", "insert widget", "drop widget", "replace widget",
 			"popup width", "popup height", "popup resizable",
 		},
-		Syntax:  "ALTER PAGE Module.Name {\n  SET property = value ON widgetName;\n  SET (prop1 = val1, prop2 = val2) ON widgetName;\n  SET Title = 'New Title';  -- page-level (case-sensitive)\n  SET Class = 'css-class';  -- page-level CSS class / style\n  SET Style = 'css: rule';\n  SET PopupWidth = 800;     -- page-level pop-up dimensions\n  SET PopupHeight = 480;\n  SET PopupResizable = true;\n  INSERT AFTER widgetName { <widgets> };\n  INSERT BEFORE widgetName { <widgets> };\n  DROP WIDGET name1, name2;\n  REPLACE widgetName WITH { <widgets> };\n};",
+		Syntax:  "ALTER PAGE Module.Name {\n  SET property = value ON widgetName;\n  SET (prop1 = val1, prop2 = val2) ON widgetName;\n  SET Title = 'New Title';  -- page-level (case-sensitive)\n  SET Class = 'css-class';  -- page-level CSS class / style\n  SET Style = 'css: rule';\n  SET PopupWidth = 800;     -- page-level pop-up dimensions\n  SET PopupHeight = 480;\n  SET PopupResizable = true;\n  INSERT AFTER widgetName { <widgets> };\n  INSERT BEFORE widgetName { <widgets> };\n  INSERT INTO containerName { <widgets> };\n  DROP WIDGET name1, name2;\n  REPLACE widgetName WITH { <widgets> };\n};",
 		Example: "ALTER PAGE Module.EditPage {\n  SET (Caption = 'Save & Close', ButtonStyle = Success) ON btnSave;\n  INSERT AFTER txtName {\n    TEXTBOX txtMiddleName (Label: 'Middle Name', Binds: MiddleName)\n  };\n  DROP WIDGET txtUnused;\n};",
 		SeeAlso: []string{"page.create", "page.show", "snippet.alter"},
 	})
@@ -152,7 +152,7 @@ func init() {
 		Keywords: []string{
 			"alter snippet", "modify snippet", "update snippet",
 		},
-		Syntax:  "ALTER SNIPPET Module.Name {\n  SET property = value ON widgetName;\n  INSERT AFTER widgetName { <widgets> };\n  INSERT BEFORE widgetName { <widgets> };\n  DROP WIDGET name1, name2;\n  REPLACE widgetName WITH { <widgets> };\n};",
+		Syntax:  "ALTER SNIPPET Module.Name {\n  SET property = value ON widgetName;\n  INSERT AFTER widgetName { <widgets> };\n  INSERT BEFORE widgetName { <widgets> };\n  INSERT INTO containerName { <widgets> };\n  DROP WIDGET name1, name2;\n  REPLACE widgetName WITH { <widgets> };\n};",
 		Example: "ALTER SNIPPET Module.NavSnippet {\n  REPLACE navItem1 WITH {\n    ACTIONBUTTON btnHome (Caption: 'Home', Action: SHOW_PAGE Module.HomePage)\n  };\n  DROP WIDGET txtOldField;\n  INSERT AFTER txtName {\n    TEXTBOX txtNewField (Label: 'New Field', Binds: NewAttr)\n  };\n};",
 		SeeAlso: []string{"snippet", "page.alter"},
 	})

docs-site/src/appendixes/quick-reference.md

@@ -421,6 +421,7 @@ Modify an existing page or snippet's widget tree in-place without full `CREATE O
 | Page-level set | `SET Title = 'New Title'` | No ON clause for page properties |
 | Insert after | `INSERT AFTER widgetName { widgets }` | Add widgets after target |
 | Insert before | `INSERT BEFORE widgetName { widgets }` | Add widgets before target |
+| Insert into | `INSERT INTO containerName { widgets }` | Append as the container's last child (fills an empty container) |
 | Drop widgets | `DROP WIDGET name1, name2` | Remove widgets by name |
 | Replace widget | `REPLACE widgetName WITH { widgets }` | Replace widget subtree |
 | Pluggable prop | `SET 'showLabel' = false ON cbStatus` | Quoted name for pluggable widgets |

docs-site/src/examples/alter-page.md

@@ -20,6 +20,18 @@ ALTER PAGE CRM.Customer_Edit {
 };
 ```
 
+## Add a Widget Into a Container
+
+Use `INSERT INTO` to append a widget as a container's last child — the only way to fill an empty container.
+
+```sql
+ALTER PAGE CRM.Customer_Overview {
+  INSERT INTO ctnToolbar {
+    ACTIONBUTTON btnNew (Caption: 'New Customer', Action: NOTHING, ButtonStyle: Primary)
+  }
+};
+```
+
 ## Remove Widgets
 
 ```sql

docs-site/src/language/alter-page.md

@@ -110,8 +110,17 @@ ALTER PAGE Module.EditPage {
     ACTIONBUTTON btnPreview (Caption: 'Preview', Action: MICROFLOW Module.ACT_Preview)
   }
 };
+
+-- Insert INTO a container — append as its last child (works on an empty container)
+ALTER PAGE Module.EditPage {
+  INSERT INTO ctnToolbar {
+    ACTIONBUTTON btnNew (Caption: 'New', Action: NOTHING, ButtonStyle: Primary)
+  }
+};
 ```
 
+`INSERT INTO <container>` appends widgets as the container's last children — the only way to fill an **empty** container, and handy for adding to a container or data view without a sibling to anchor to. Widgets inserted into a data view take that data view's entity. Supported on simple containers; for a layout grid or tab container, insert relative to a widget inside the target column/tab instead.
+
 The inserted widgets use the same syntax as in `CREATE PAGE`. Multiple widgets can be inserted in a single block.
 
 ### DROP WIDGET -- Remove Widgets

docs-site/src/language/snippets.md

@@ -97,6 +97,9 @@ ALTER SNIPPET MyModule.CustomerCard {
   SET Caption = 'View Details' ON btnEdit;
   INSERT AFTER txtEmail {
     DYNAMICTEXT txtPhone (Content: '{1}', Attribute: Phone)
+  };
+  INSERT INTO ctnActions {   -- append as the container's last child
+    ACTIONBUTTON btnMore (Caption: 'More', Action: NOTHING)
   }
 };
 ```

docs-site/src/reference/page/alter-page.md

@@ -29,6 +29,9 @@ SET 'propertyName' = value ON widgetName;
 INSERT BEFORE widgetName { widget_definitions };
 INSERT AFTER widgetName { widget_definitions };
 
+-- Insert widgets as the last children of a container
+INSERT INTO containerName { widget_definitions };
+
 -- Remove widgets
 DROP WIDGET widgetName1, widgetName2;
 
@@ -68,6 +71,12 @@ For pluggable widget properties, use quoted property names (e.g., `'showLabel'`)
 
 Inserts new widgets immediately before or after a named widget within its parent container. The new widgets use the same syntax as in `CREATE PAGE`.
 
+### INSERT INTO
+
+Appends new widgets as the **last children** of a named container. This is the only way to fill an **empty** container, and the natural way to add a widget to a container or data view without needing a sibling to anchor to. Widgets inserted into a data view take that data view's entity as their context.
+
+Supported on simple containers (container/DivContainer, data view, group box, scroll-container region). A layout grid (rows/columns) and tab container have no single child list — insert relative to a widget inside the target column or tab instead.
+
 ### DROP WIDGET
 
 Removes one or more widgets by name. The widget and all its children are removed from the tree.

docs/01-project/MDL_QUICK_REFERENCE.md

@@ -982,6 +982,7 @@ Modify an existing page or snippet's widget tree in-place without full `create o
 | Widget dynamic classes | `set DynamicClasses = 'expr' on widgetName` | Runtime-computed classes on a widget — the surgical alternative to a bulk `update widgets` |
 | Insert after | `insert after widgetName { widgets }` | Add widgets after target |
 | Insert before | `insert before widgetName { widgets }` | Add widgets before target |
+| Insert into | `insert into containerName { widgets }` | Append as the container's last child (fills an empty container; dataview children take its entity) |
 | Drop widgets | `drop widget name1, name2` | Remove widgets by name |
 | Replace widget | `replace widgetName with { widgets }` | Replace widget subtree |
 | Pluggable prop | `set 'showLabel' = false on cbStatus` | Quoted name for pluggable widgets |

docs/05-mdl-specification/01-language-reference.md

@@ -838,6 +838,7 @@ set title = 'New Title';
 -- Insert widgets
 insert after widgetName { <widgets> };
 insert before widgetName { <widgets> };
+insert into containerName { <widgets> };   -- append as the container's last child
 
 -- Remove widgets
 drop widget name1, name2;

mdl-examples/doctype-tests/33-alter-page-examples.mdl

@@ -131,6 +131,19 @@ alter page AlterPg.Product_Overview {
   }
 };
 
+-- MARK: AP02b — INSERT INTO a container (append as last child)
+
+-- ============================================================================
+-- AP02b: INSERT INTO — append widgets as a container's children. This is the
+-- only way to fill an EMPTY container (here, the `divider` just created above).
+-- ============================================================================
+
+alter page AlterPg.Product_Overview {
+  insert into divider {
+    dynamictext dividerNote (Content: 'Catalog below', RenderMode: paragraph)
+  }
+};
+
 -- MARK: AP03 — INSERT a DataGrid2 with initial columns
 
 -- ============================================================================