CREATE LAYOUT — the frame a page is built on

A layout is the frame every page renders inside: the topbar, the navigation sidebar, and the hole the page's own content drops into. Until now it was the one document mxcli could not author, which put the topbar out of reach of MDL.

Do not edit Atlas_Core

Mendix's own documentation says it plainly: "Do not change the supplied layouts. Either create a separate module with the custom layouts, page templates, and building blocks or create your own." An Atlas layout lives in a Marketplace module, and a Marketplace update replaces the module wholesale — every local edit is gone, silently.

CREATE LAYOUT refuses a Marketplace module for that reason. Put your layout in a module you own.

The usual starting point is a copy of an Atlas layout, and describe already gives you one:

mxcli -p app.mpr -c "describe layout Atlas_Core.Atlas_Default" > mine.mdl
# edit the qualified name to your own module, then:
mxcli exec mine.mdl -p app.mpr

DESCRIBE LAYOUT emits re-executable MDL, so describe → rename → exec is the copy operation. There is no COPY DOCUMENT verb and none is needed.

A copy is only as good as what MDL can spell. A widget describe cannot render comes out as a comment ending -- NOT re-executable, and re-running the script drops it. Measured on Atlas_Core.Atlas_SideBar: its two Forms$SidebarToggleButton widgets do not survive the round trip, and an image widget copied this way loses its image reference (CE0463 until mxcli fix widgets, then "No image selected"). Read the describe output before running it — the comments name exactly what will be lost. To change a layout without that risk, use ALTER LAYOUT, which edits the stored document in place and leaves everything it was not asked to touch alone.

Removing a layout

DROP LAYOUT MyModule.App_Old;

Pages still bound to it are named in a warning and the drop proceeds — it is not refused. Left dropped, each of those pages fails the build with CE1613 ("The selected layout … no longer exists"), which names the page and never the layout, so repoint them first:

ALTER PAGES SET LAYOUT = MyModule.App_New WHERE LAYOUT = MyModule.App_Old;
DROP LAYOUT MyModule.App_Old;

To correct a layout rather than remove it, re-create it under the same name (CREATE OR REPLACE LAYOUT): the pages stay bound by qualified name and rebind to the new document — verified end to end, the pages go back to 0 errors.

Repointing pages

A new layout that no page uses changes nothing:

-- one page
alter page MyModule.Home { set Layout = MyModule.App_Default; };

-- the migration: every page currently on Atlas_Default
alter pages set layout = MyModule.App_Default
  where layout = Atlas_Core.Atlas_Default;

-- scoped to one module, whatever each page is on now
alter pages in MyModule set layout = MyModule.App_Default;

Both rewrite the layout reference and every placeholder binding. Pages in Marketplace modules are skipped and named — they are not yours to edit either.

A page bound to a placeholder the new layout does not declare is refused before anything is written; map (Old as New) is how you rebind it:

alter page MyModule.Split { set Layout = MyModule.Minimal map (HeaderLeft as Main); };

ALTER LAYOUT

Edits the stored document rather than rewriting it, so widgets MDL cannot spell survive. Same operations as ALTER PAGE:

alter layout MyModule.App_Default {
  insert into layoutContainer.top { snippetcall bar (snippet: MyModule.SNIPPET_ThemeBar) };
  set Content = 'My App' on brandText;
  drop widget oldBanner;
};

A region has no name of its own — its slot is its identity — so it is addressed as <scrollContainerName>.<slot>, reusing the dotted widget reference. Only INSERT INTO takes a region; BEFORE/AFTER position a widget among siblings, so name the widget instead. An empty slot has no stored region document to insert into: add it with create or replace layout.

ALTER LAYOUT refuses a Marketplace target, and names the copy-then-repoint route in the error.

Syntax

create [or replace] layout MyModule.App_Default (
  layouttype: 'Responsive',
  class: 'layout-atlas layout-atlas-responsive-topbar'
) {
  scrollcontainer layoutContainer {
    region top (size: 60, sizemode: 'Fixed', class: 'region-topbar') {
      snippetcall topbar (snippet: MyModule.SNIPPET_TopBar)
    }
    region left (size: 232, sizemode: 'Pixels', class: 'region-sidebar') {
      navigationtree navMenu (profile: 'Responsive')
    }
    region center (class: 'region-content') {
      placeholder Main
    }
  }
}

A page then names the layout, and its widgets land in the Main placeholder:

create page MyModule.Home (title: 'Home', layout: MyModule.App_Default) {
  dynamictext welcome (content: 'Hello')
}

The four things only a layout has

Element	MDL	Notes
Scroll container	scrollcontainer name { … }	The layout's root. Its children are regions, never widgets
Region	region top | right | bottom | left | center	Five named slots, not a list. One region per slot; a repeat is refused
Placeholder	placeholder Main	The hole a page's content goes into. No properties, no body
Navigation tree	navigationtree name (profile: 'Responsive')	The sidebar menu — vertical. The profile is a navigation profile name
Menu bar	menubar name (profile: 'Responsive')	The topbar menu — horizontal. Same stored shape as a navigation tree

Region properties: size (integer), sizemode (Fixed / Pixels / Auto), class. Unset is Studio Pro's 200 / Auto.

Layout type, and why there is no native: flag

Platform	Values
Web	Responsive, Phone, Tablet, ModalPopup
Native	Default, Popup

Measured across all 22 layouts Atlas ships. The two sets are disjoint, so the platform is inferred from the type — a native: property could only ever contradict it. A cross-platform value (Default on a web layout) is refused, not silently accepted.

The layout class is load-bearing

class: sets the layout's own CSS class, and Atlas scopes ~24 of its layout rules to .layout-atlas and its variants. Every Atlas layout with chrome carries one — layout-atlas layout-atlas-responsive-topbar, layout-atlas layout-atlas-responsive-default, and so on; only PopupLayout, which has no chrome, is bare.

Leave it off and the layout builds cleanly, passes mx check, and renders with no topbar bar and no sidebar rail — a difference only a browser shows. Use the Atlas class that matches the shape you are building:

Shape	Class
Topbar navigation	layout-atlas layout-atlas-responsive-topbar
Sidebar navigation	layout-atlas layout-atlas-responsive-default
Popup	(none)

layouttype, class and style are the only header properties. Anything else is an error rather than an ignored key, which is the point of the next section.

Gotchas

• A placeholder's name is API. A page binds to it as Module.Layout.<Name>, stored as a qualified name. Renaming one unbinds every page that used it — the page still builds, and its content vanishes.

• Exactly one placeholder must be named Main — this is a rule, not a convention. Forms$Layout has no property saying which placeholder is the main one, but mxbuild validates the NAME. Measured on 11.12.1 against a layout no page uses, so none of it depends on a page binding:

  Declares	mxbuild
  Main	0 errors
  Main + Content	0 errors — extra names are fine
  Content only	CE0848 "No placeholder with the name 'Main' found. There should be exactly one."
  Main + Main	CE0849 + CE0495
  Main + Side + Side	CE0495 "Duplicate name 'Side'." — uniqueness is general

  mxcli check reports these as MDL081 (the Main rule) and MDL082 (duplicate names). Earlier versions said this was a convention and checked only that some placeholder existed, so a layout naming it anything else passed check and exec and failed the build (mendixlabs/mxcli#1063).

• There is no mainplaceholder: property, on purpose. modelsdk/gen declares MainPlaceholderName on Layout so the setter compiles, and mxbuild accepts the result — measured 0 errors. But generated/metamodel does not declare it and no Studio Pro layout carries it, and Studio Pro resolves every stored property against the type's list. Writing it gives you a layout that builds and cannot be opened.

• A placeholder is declared with NO body. placeholder Main { … } is the page-side spelling — in a page it fills a layout's slot; in a layout it declares nothing and is dropped, leaving the layout with no placeholder at all. Reaching for it here is the natural mistake, since every other layout element takes a body. Reported as MDL083.

• A layout that declares no placeholder is refused at write time as well as at check time — no page could use it.

• The sidebar toggle, the menu bar's logo and Atlas's Forms$Header are not authorable. A topbar layout that needs a collapsible sidebar therefore has to keep Atlas's, or do without the toggle — which is why mxcli new scaffolds a topbar-navigation layout with no sidebar rather than an always-open one.

• Authoring is modelsdk-only. The legacy writer cannot produce the Content wrapper the widget tree hangs off; it refuses rather than writing a layout with no tree.

• create or replace layout rewrites the whole document; alter layout does not. Prefer ALTER for an edit to a layout you did not author from MDL — a rewrite is only ever as complete as the describe it came from.

Validate

mxcli check layout.mdl                       # syntax
mxcli check layout.mdl -p app.mpr --references
mxcli exec layout.mdl -p app.mpr
mxcli -p app.mpr -c "describe layout MyModule.App_Default"   # round-trip

A layout is a rendering artifact, so a clean mx check proves little on its own. To see it, boot the app: mxcli run --local -p app.mpr --screenshot and look at the regions.

What mxcli new scaffolds

A new project gets <YourModule>.App_Default — this layout, in a module it owns — and its pages moved onto it, so the documented practice is the default rather than something to discover. mxcli new --layout none keeps Atlas's.

It is not a copy of Atlas_TopBar, and could not be: that layout carries a Forms$MenuBar (now authorable), a Forms$SidebarToggleButton and a pluggable image, so a describe → exec copy renders with no navigation and no logo. The scaffold reproduces the result instead — same layout class, same region classes, navigation in the topbar, Main for page content — and omits the toggle button and the stock logo.

Layouts (SHOW/DESCRIBE/CREATE [OR REPLACE] LAYOUT)

the frame a page renders inside — the last document a page depends on that MDL could not write, which is why the topbar was out of reach. Four element types beyond a page's vocabulary: scrollcontainer, region top|right|bottom|left|center (five named slots, not a list), placeholder, navigationtree. DESCRIBE LAYOUT emits re-executable MDL, which makes describe → rename → exec the copy operation and is why there is no COPY DOCUMENT verb. Writing into a Marketplace module is refused — Mendix's own guidance is not to edit the supplied layouts, since an update replaces the module — which also required wiring FromAppStore enrichment into GetModuleByName/GetModule (it was populated only by ListModules, so the guard would have been inert). The document is pinned to the ten keys Studio Pro writes, identical across all 22 Atlas layouts on 11.13.0: modelsdk/gen offers seven placeholder properties on Layout (MainPlaceholderName and friends) that generated/metamodel does not declare and no real layout carries, and writing one gives a layout mxbuild accepts at 0 errors and Studio Pro cannot open. Which placeholder is "main" is a rule mxbuild enforces, not a convention — layouttype is still the only header property, because Forms$Layout has no property for it, but the NAME is validated: measured on 11.12.1 against a layout no page uses, exactly one placeholder must be named Main (none → CE0848, two → CE0849) and placeholder names must be unique (CE0495); extra placeholders under other names are fine. mxcli reported none of this until #1063 — this file called it a convention and the write-time guard implemented that belief, accepting any placeholder under any name, so the reported script passed check AND exec and failed a build later with no DROP LAYOUT to undo it. The rule now lives once in types.CheckLayoutPlaceholderNames and is applied by both mxcli check (MDL081/MDL082) and the writer, because two copies in two currencies is how a resolver drifts. The platform is inferred from the layout type — web (Responsive/Phone/Tablet/ModalPopup) and native (Default/Popup) are disjoint — so there is no native: flag. Authoring is modelsdk-only; legacy refuses. Verified in a browser, not just against mx check. DROP LAYOUT Module.Name removes one — layouts were the only doctype mxcli could create and alter but not delete, which bit hardest on a layout mxcli itself had just written badly; pages still bound to it are warned about and named, not refused (matching every other DROP), because drop-then-recreate under the same name is how a layout is corrected and the pages rebind by qualified name — left dropped they fail CE1613, which names the page and never the layout. ALTER LAYOUT Module.Name { … } takes the whole ALTER PAGE vocabulary (a layout's tree is a page's plus four element types) and edits the stored document, which is what makes it a capability rather than a convenience: describe → rename → exec is only as complete as what MDL can spell — measured, the copy of Atlas_Core.Atlas_SideBar loses both Forms$SidebarToggleButton widgets, and an image widget loses its image reference. A placeholder is declared with no body; placeholder X { … } is the page-side spelling that fills a slot, and in a layout the visitor drops it — which used to leave the layout with no placeholder at all and fail the write with a message contradicting the script, so it is now MDL083. A region has no Name, so it is addressed as layoutContainer.top (the dotted widgetRef that also serves DataGrid2 columns; $Type decides which), and only INSERT INTO takes one. Pages move onto a layout with ALTER PAGE … SET Layout = X [MAP (Old AS New)] or the bulk ALTER PAGES [IN <mod>] SET LAYOUT = X [WHERE LAYOUT = Y], both of which refuse a repoint that would leave a page bound to a placeholder the target does not declare (checked after MAP, since MAP is the remedy) — mxbuild only catches that as CE1613 at the far end of a build. and docs/11-proposals/PROPOSAL_authorable_layouts.md