@@ -0,0 +1,22 @@
+---
+'@objectstack/spec': minor
+---
+
+feat(spec)!: `deriveRecordFlowSurface` derives a bounded modal (`size: 'auto'`) for a field-heavy create / edit / child task flow on desktop; a mobile posture keeps the full-screen modal (`size: 'full'`)
+
+A task flow (`create`, `edit`, `child-create`, `child-edit`) whose base surface is `'page'` (at or above `RECORD_SURFACE_PAGE_THRESHOLD` authorable fields) still opens as an overlay modal, but on desktop its size is now `'auto'`, so the renderer's own `lg` / `xl` bound applies instead of stretching the form to the window edge. With `viewport: 'mobile'` the derivation answers `size: 'full'` exactly as before. A heavy form with an embedded child or line-items table is not split out: it takes the same bounded modal on desktop. The `view` flow and every light-object flow are unchanged.
+
+Clause-②: no
+
+<!-- adr-0087: not-required (no-migration-prescription) No metadata moves: no spec key, authorable spelling or stored shape is added, removed, renamed or re-shaped, and no export is added, removed or narrowed (`RecordFlowSurface.size` was already `'auto' | 'full'`). What changes is the value a published pure function derives for one input class, a desktop field-heavy task flow, so there is nothing for `objectstack migrate meta` to rewrite. The remedy for a host that wants the old size is an explicit `navigation.size` / `FormView.modalSize`, never a rewrite of a package source or a stored row. -->
+
+**BREAKING** (a published derivation's default changes), shipped as `minor` under the launch-window convention for breaking changes.
+
+What changes for you: a consumer that relied on `deriveRecordFlowSurface(def, flow)` answering `size: 'full'` for a field-heavy task flow on desktop now gets `size: 'auto'`.
+
+| call | FROM | TO |
+|:--|:--|:--|
+| `deriveRecordFlowSurface(heavyDef, 'create')` (also `'edit'`, `'child-create'`, `'child-edit'`; or `{ viewport: 'desktop' }`) | `{ container: 'overlay', surface: 'modal', size: 'full' }` | `{ container: 'overlay', surface: 'modal', size: 'auto' }` |
+| the same call with `{ viewport: 'mobile' }` | `{ container: 'overlay', surface: 'modal', size: 'full' }` | unchanged |
+
+The one-line fix, for an object that genuinely needs the full-screen form on desktop: set `navigation.size: 'full'` on its list view (or `FormView.modalSize: 'full'`), which wins over the derived default.
@@ -70,9 +70,26 @@ describe('deriveRecordFlowSurface — viewing a record may route, a create or ed
     });
   });
 
-  it('task flows never route: heavy → full-screen modal overlay', () => {
+  it('task flows never route: heavy on desktop → a bounded modal overlay (size auto), not full-screen', () => {
     for (const flow of TASK_FLOWS) {
       expect(deriveRecordFlowSurface(heavy, flow)).toEqual({
+        container: 'overlay', surface: 'modal', size: 'auto',
+      });
+      expect(deriveRecordFlowSurface(heavy, flow, { viewport: 'desktop' })).toEqual({
+        container: 'overlay', surface: 'modal', size: 'auto',
+      });
+    }
+  });
+
+  it('a heavy form with an embedded line-items table takes the same bounded modal on desktop (no subtable split)', () => {
+    const withLineItems = objWithFields(RECORD_SURFACE_PAGE_THRESHOLD, {
+      line_items: { type: 'repeater', label: 'Line items', fields: { sku: { type: 'text' } } },
+    });
+    for (const flow of TASK_FLOWS) {
+      expect(deriveRecordFlowSurface(withLineItems, flow)).toEqual({
+        container: 'overlay', surface: 'modal', size: 'auto',
+      });
+      expect(deriveRecordFlowSurface(withLineItems, flow, { viewport: 'mobile' })).toEqual({
         container: 'overlay', surface: 'modal', size: 'full',
       });
     }
@@ -86,28 +103,33 @@ describe('deriveRecordFlowSurface — viewing a record may route, a create or ed
     }
   });
 
-  it('mobile: view routes to a page; task flows become a full-screen modal', () => {
+  it('mobile: view routes to a page; task flows, light or heavy, become a full-screen modal', () => {
     expect(deriveRecordFlowSurface(light, 'view', { viewport: 'mobile' })).toEqual({
       container: 'route', surface: 'page', size: 'auto',
     });
     for (const flow of TASK_FLOWS) {
       expect(deriveRecordFlowSurface(light, flow, { viewport: 'mobile' })).toEqual({
         container: 'overlay', surface: 'modal', size: 'full',
       });
+      expect(deriveRecordFlowSurface(heavy, flow, { viewport: 'mobile' })).toEqual({
+        container: 'overlay', surface: 'modal', size: 'full',
+      });
     }
   });
 
   it('child-* flows size to the def they are given (the child), independent of any parent', () => {
     // A thin child stays a drawer even though its parent (not passed) is heavy.
     expect(deriveRecordFlowSurface(objWithFields(3), 'child-create').surface).toBe('drawer');
-    // A fat child gets the full-screen modal.
+    // A fat child gets the bounded modal on desktop.
     expect(deriveRecordFlowSurface(objWithFields(40), 'child-edit')).toEqual({
-      container: 'overlay', surface: 'modal', size: 'full',
+      container: 'overlay', surface: 'modal', size: 'auto',
     });
   });
 
   it('honours pageThreshold and tolerates bare/malformed input', () => {
-    expect(deriveRecordFlowSurface(objWithFields(5), 'create', { pageThreshold: 4 }).size).toBe('full');
+    expect(deriveRecordFlowSurface(objWithFields(5), 'create', { pageThreshold: 4 })).toEqual({
+      container: 'overlay', surface: 'modal', size: 'auto',
+    });
     expect(deriveRecordFlowSurface(null, 'create')).toEqual({
       container: 'overlay', surface: 'drawer', size: 'auto',
     });
 
@@ -128,9 +128,24 @@ export interface RecordFlowSurface {
  *     state → route-capable; making/changing one is a task → always overlay.
  *
  * So `view` keeps the #2578 behavior verbatim (`'page'` → route), while the
- * task flows map the derived `'page'` to a FULL-SCREEN MODAL — same big
- * canvas, overlay return semantics. This mapping is why `'modal'` exists in
- * {@link RecordSurface} without the base heuristic ever emitting it.
+ * task flows map the derived `'page'` to a MODAL — overlay return semantics.
+ * This mapping is why `'modal'` exists in {@link RecordSurface} without the
+ * base heuristic ever emitting it. How big that modal is follows the viewport:
+ *   - desktop (the default): a BOUNDED modal, `size: 'auto'` — the renderer's
+ *     own `lg` / `xl` bound applies, so a field-heavy form is not stretched to
+ *     the window edge;
+ *   - `viewport: 'mobile'`: a FULL-SCREEN modal, `size: 'full'` (an overlay on
+ *     a phone has no room to be anything else).
+ * The split is by viewport only. A field-heavy form with an embedded child /
+ * line-items table takes the same bounded modal on desktop: #2604 D1 sized
+ * task flows by field count alone and scoped the in-form `inlineEdit` grid out
+ * of its design, so it never chose full-screen for that case.
+ *
+ * #2604 D1 (2026-07-05) mapped a field-heavy task flow to a full-screen modal
+ * on every viewport.
+ * Amended 2026-10-15 by objectui#12521 ruling 6203721639 (letter B): on
+ * desktop the field-heavy task flow is a bounded modal; mobile keeps
+ * full-screen.
  *
  * Like the base derivation this is a DEFAULT only: explicit `navigation.mode`
  * / `navigation.size`, `FormView.type` / `modalSize`, or an assigned page win
@@ -145,8 +160,11 @@ export function deriveRecordFlowSurface(
   if (flow === 'view') {
     return { container: surface === 'page' ? 'route' : 'overlay', surface, size: 'auto' };
   }
-  // Task flows (create / edit / child-*): never a route. Field-heavy (or
-  // mobile, where the base derivation says 'page') → full-screen modal.
-  if (surface === 'page') return { container: 'overlay', surface: 'modal', size: 'full' };
+  // Task flows (create / edit / child-*): never a route. The base derivation
+  // says 'page' when field-heavy or mobile → a modal: full-screen on mobile,
+  // bounded (`'auto'`, the renderer's lg/xl bound) on desktop.
+  if (surface === 'page') {
+    return { container: 'overlay', surface: 'modal', size: opts.viewport === 'mobile' ? 'full' : 'auto' };
+  }
   return { container: 'overlay', surface, size: 'auto' };
 }