diff --git a/.changeset/gantt-navconfig-comment-once.md b/.changeset/gantt-navconfig-comment-once.md
new file mode 100644
index 0000000000..0390f51d96
--- /dev/null
+++ b/.changeset/gantt-navconfig-comment-once.md
@@ -0,0 +1,15 @@
+---
+---
+
+`ObjectGantt` internal cleanup, measured as a zero-pixel change:
+
+- The comment block above the `navConfig` default repeated its own last two
+  lines verbatim, leaving a mid-sentence fragment. The sentence is now stated
+  once, in full.
+- The drawer default no longer spells the spec-deprecated `width`
+  (`@deprecated [#2578 -> size]`). It is now `{ mode: 'drawer' }`, so
+  `resolveOverlayWidth` returns `undefined` and `RecordDetailDrawer`'s own
+  `width` default supplies the identical `min(960px, 60vw)`. The resolved
+  overlay width is unchanged on every viewport, and is now pinned by a test.
+
+No published behaviour changes.
diff --git a/packages/plugin-gantt/src/ObjectGantt.navWidthDefault.test.tsx b/packages/plugin-gantt/src/ObjectGantt.navWidthDefault.test.tsx
new file mode 100644
index 0000000000..28f37a60d3
--- /dev/null
+++ b/packages/plugin-gantt/src/ObjectGantt.navWidthDefault.test.tsx
@@ -0,0 +1,106 @@
+/**
+ * Pins the drawer width a gantt gets when it declares no `navigation`.
+ *
+ * ObjectGantt used to hard-code `{ mode: 'drawer', width: 'min(960px, 60vw)' }`
+ * as that default. `width` is `@deprecated [#2578 -> size]` and
+ * `resolveOverlayWidth` gives an explicit `width` priority OVER `size`, so
+ * spelling it kept the deprecated branch load-bearing on the path most gantts
+ * take. The default is now `{ mode: 'drawer' }`: `resolveOverlayWidth` returns
+ * `undefined` and RecordDetailDrawer's own `width` default supplies the
+ * identical CSS — a zero-pixel change.
+ *
+ * That equivalence was previously pinned by NOTHING: a repo-wide search for
+ * `min(960px, 60vw)` returned only producers, zero assertions, and the whole
+ * 402-test gantt suite stayed green when the value was changed. Both halves
+ * below are load-bearing and fail for different reasons:
+ *
+ *   half 1 — the gantt must stop injecting a width of its own (it has to hand
+ *            `undefined` down, or the drawer's default can never apply);
+ *   half 2 — the width the REAL drawer then resolves must still be that value.
+ *            Without this half the gantt would follow a moved drawer default
+ *            invisibly, which is the regression the indirection introduces.
+ *
+ * Both assert the resolved width VALUE — never a `className`, never "it
+ * renders", either of which passes in both worlds.
+ */
+import React from 'react';
+import { render, screen, waitFor, fireEvent } from '@testing-library/react';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { ObjectGantt } from './ObjectGantt';
+
+/** The width the gantt's drawer has always resolved to. Must not drift. */
+const EXPECTED_WIDTH = 'min(960px, 60vw)';
+
+vi.mock('sonner', () => ({ toast: { error: vi.fn() } }));
+
+vi.mock('./GanttView', () => ({
+  GanttView: ({ tasks, onTaskClick }: any) => (
+    <div data-testid="gantt-view">
+      {tasks.map((t: any) => (
+        <button key={t.id} data-testid={`gv-view-${t.id}`} onClick={() => onTaskClick?.(t)}>
+          {t.title}
+        </button>
+      ))}
+    </div>
+  ),
+}));
+
+// Record the props ObjectGantt hands down, then delegate to the REAL drawer so
+// half 2 measures the actual resolution rather than a stub's idea of it.
+let drawerProps: any = null;
+vi.mock('@object-ui/plugin-detail', async (importOriginal) => {
+  const actual = await importOriginal<any>();
+  const Real = actual.RecordDetailDrawer;
+  return {
+    ...actual,
+    RecordDetailDrawer: (props: any) => {
+      drawerProps = props;
+      return <Real {...props} />;
+    },
+  };
+});
+
+function makeSchema(): any {
+  return {
+    type: 'gantt',
+    objectName: 'tasks',
+    gantt: { titleField: 'name', startDateField: 'start_date', endDateField: 'end_date' },
+    data: {
+      provider: 'value',
+      items: [{ id: '1', name: 'Row', start_date: '2024-01-01', end_date: '2024-01-05' }],
+    },
+  };
+}
+
+async function openDrawer() {
+  render(<ObjectGantt schema={makeSchema()} />);
+  await waitFor(() => expect(screen.getByTestId('gv-view-1')).toBeDefined());
+  fireEvent.click(screen.getByTestId('gv-view-1'));
+  await waitFor(() => expect(drawerProps).not.toBeNull());
+}
+
+describe('gantt drawer width with no declared `navigation`', () => {
+  beforeEach(() => {
+    drawerProps = null;
+    // Cross-test leakage guard: the drawer prefers a drag-resized width
+    // persisted in localStorage over its prop, which would mask half 2.
+    try { window.localStorage.clear(); } catch { /* ignore */ }
+  });
+
+  it('half 1: the gantt injects no width of its own (so the drawer default applies)', async () => {
+    await openDrawer();
+    expect(drawerProps.width).toBeUndefined();
+  });
+
+  it('half 2: the width the real drawer resolves is still the pinned value', async () => {
+    await openDrawer();
+    // The drawer applies the resolved width as an inline style on its panel,
+    // as BOTH `width` and `max-width`. happy-dom's CSS parser drops the
+    // `width` longhand when the value is a `min()` expression but keeps
+    // `max-width`, so the surviving declaration is what we read — it is the
+    // same resolved string, not a proxy for it.
+    const panel = document.querySelector('[role="dialog"]') as HTMLElement | null;
+    expect(panel, 'drawer panel').not.toBeNull();
+    expect(panel!.style.maxWidth).toBe(EXPECTED_WIDTH);
+  });
+});
diff --git a/packages/plugin-gantt/src/ObjectGantt.tsx b/packages/plugin-gantt/src/ObjectGantt.tsx
index 22d30b9510..ae81c5c22e 100644
--- a/packages/plugin-gantt/src/ObjectGantt.tsx
+++ b/packages/plugin-gantt/src/ObjectGantt.tsx
@@ -1147,9 +1147,24 @@ export const ObjectGantt: React.FC<ObjectGanttProps> = ({
   // Default to a right-side drawer so clicking a task opens an editable
   // detail panel inline (no full-page navigation). Schema can override by
   // providing its own `navigation` config (e.g., page mode).
-  // detail panel inline (no full-page navigation). Schema can override by
-  // providing its own `navigation` config (e.g., page mode).
-  const navConfig = schema.navigation ?? { mode: 'drawer', width: 'min(960px, 60vw)' };
+  //
+  // No width is spelled here on purpose. `width` is `@deprecated [#2578 ->
+  // size]` in the spec that owns this shape, and `resolveOverlayWidth` gives
+  // an explicit `width` priority OVER `size` — so spelling it kept the
+  // deprecated branch load-bearing on the path most gantts take (no declared
+  // `navigation`), and made the size buckets unreachable there. Omitting both
+  // leaves `resolveOverlayWidth` returning `undefined`, which is what
+  // RecordDetailDrawer's own `width` default is for; that default is the
+  // identical `min(960px, 60vw)`, so this is a zero-pixel change on every
+  // viewport. Pinned by ObjectGantt.navWidthDefault.test.tsx — both halves,
+  // because the equivalence now depends on the drawer's default too.
+  //
+  // Deliberately NOT converged on `size: 'lg'`: that bucket is
+  // `min(92vw, 960px)`, which agrees with the above only at viewport >=
+  // 1600px and is up to 53% wider below it. Whether the renderers should move
+  // to the bucket is open (#6259), and spans kanban/calendar/RecordDetailDrawer
+  // as one decision (#6303).
+  const navConfig = schema.navigation ?? { mode: 'drawer' };
   const navIsOverlay = navConfig.mode === 'drawer' || navConfig.mode === 'modal' || navConfig.mode === 'split' || navConfig.mode === 'popover';
   const navigation = useNavigationOverlay({
     navigation: navConfig,