diff --git a/packages/components/src/__tests__/resizable-orientation.test.tsx b/packages/components/src/__tests__/resizable-orientation.test.tsx
new file mode 100644
index 000000000..b1942e2fd
--- /dev/null
+++ b/packages/components/src/__tests__/resizable-orientation.test.tsx
@@ -0,0 +1,133 @@
+/**
+ * ObjectUI
+ * Copyright (c) 2024-present ObjectStack Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * The stacked resizable divider — `custom/resizable.tsx`.
+ *
+ * `ui/resizable.tsx` is the react-resizable-panels **v3** Shadcn file: its
+ * whole stacked-group appearance hangs off `data-[panel-group-direction=vertical]`,
+ * an attribute v4 (the installed major) never emits. So in a stacked group the
+ * divider kept the base `w-px` and came out a 1px-wide, 16px-tall sliver — its
+ * only height being the grip — instead of a full-width hairline, with a 4x16px
+ * drag target pinned to the group's left edge and an unrotated grip.
+ * `custom/resizable.tsx` re-keys those styles onto `aria-orientation`, which v4
+ * does emit on the separator.
+ *
+ * These tests pin the *class contract* and the attribute it depends on; happy-dom
+ * has no Tailwind, so resolved geometry (478x1px rule, 478x4px hit strip, grip at
+ * `rotate: 90deg`, and a real off-grip drag moving the split 50 → 75.21) is
+ * verified in a browser instead — see the PR.
+ *
+ * The load-bearing case is the one that reads backwards: `aria-orientation`
+ * describes the SEPARATOR, so a `orientation="vertical"` group (stacked panels)
+ * has a `aria-orientation="horizontal"` divider. Keying the stacked styles on
+ * `aria-[orientation=vertical]` would be the easy, silent way to reintroduce
+ * exactly this bug — hence the first test.
+ */
+
+import { describe, it, expect, afterEach } from 'vitest';
+import { render, cleanup } from '@testing-library/react';
+
+import {
+  ResizableHandle,
+  ResizablePanel,
+  ResizablePanelGroup,
+} from '../custom/resizable';
+
+afterEach(cleanup);
+
+function renderGroup(orientation: 'horizontal' | 'vertical') {
+  const { container } = render(
+    <ResizablePanelGroup orientation={orientation}>
+      <ResizablePanel defaultSize={50}>one</ResizablePanel>
+      <ResizableHandle withHandle />
+      <ResizablePanel defaultSize={50}>two</ResizablePanel>
+    </ResizablePanelGroup>,
+  );
+  const separator = container.querySelector<HTMLElement>('[role="separator"]');
+  if (!separator) throw new Error('no separator rendered');
+  return { container, separator };
+}
+
+/**
+ * Every `aria-[name=value]:`-prefixed utility on the element, as the
+ * [attribute, value] pair the variant actually selects on — Tailwind's `aria-[…]`
+ * variant prepends `aria-` to the bracketed name, so `aria-[orientation=horizontal]:`
+ * compiles to `&[aria-orientation="horizontal"]`.
+ */
+function ariaVariantsOn(el: HTMLElement): Array<[string, string]> {
+  return [...el.classList].flatMap((cls) => {
+    const m = /(?:^|:)aria-\[([a-z-]+)=([^\]]+)\]:/.exec(cls);
+    return m ? [[`aria-${m[1]}`, m[2]] as [string, string]] : [];
+  });
+}
+
+describe('custom/resizable — the stacked divider (react-resizable-panels v4)', () => {
+  it('gives the stacked divider aria-orientation=horizontal, not vertical', () => {
+    // The inversion that makes this bug easy to reintroduce: the group stacks
+    // vertically, so the rule between the panels runs horizontally.
+    expect(renderGroup('vertical').separator.getAttribute('aria-orientation')).toBe('horizontal');
+    expect(renderGroup('horizontal').separator.getAttribute('aria-orientation')).toBe('vertical');
+  });
+
+  it('keys the stacked divider on that attribute, so a hairline replaces the sliver', () => {
+    const { separator } = renderGroup('vertical');
+    const classes = [...separator.classList];
+
+    // The rule: full-width, 1px tall — overriding the base `w-px` that made it a
+    // vertical sliver in a stacked group.
+    expect(classes).toContain('aria-[orientation=horizontal]:h-px');
+    expect(classes).toContain('aria-[orientation=horizontal]:w-full');
+    // The `::after` drag target: a strip ALONG the rule, not across it.
+    expect(classes).toContain('aria-[orientation=horizontal]:after:h-1');
+    expect(classes).toContain('aria-[orientation=horizontal]:after:w-full');
+    expect(classes).toContain('aria-[orientation=horizontal]:after:left-0');
+    expect(classes).toContain('aria-[orientation=horizontal]:after:translate-x-0');
+    expect(classes).toContain('aria-[orientation=horizontal]:after:-translate-y-1/2');
+    // The grip crosses the rule only when rotated.
+    expect(classes).toContain('[&[aria-orientation=horizontal]>div]:rotate-90');
+  });
+
+  it('leaves the side-by-side divider on the base classes', () => {
+    const { separator } = renderGroup('horizontal');
+    // Same class string either way — nothing above matches, so the base
+    // `w-px` / `after:left-1/2` / `after:-translate-x-1/2` styling stands and
+    // the grip is not rotated. This is the case that already worked; it must
+    // keep working.
+    expect(separator.getAttribute('aria-orientation')).toBe('vertical');
+    for (const [attr, value] of ariaVariantsOn(separator)) {
+      expect(separator.matches(`[${attr}="${value}"]`)).toBe(false);
+    }
+  });
+
+  it('only keys on attributes the installed library actually emits', () => {
+    // The guard that would have caught the original bug, and catches the next
+    // rename: every `aria-[…]` variant the wrapper adds must match the real DOM
+    // in the orientation it targets. A variant naming an attribute/value the
+    // library never writes is a dead selector — which is exactly what
+    // `data-[panel-group-direction=vertical]` became when v4 landed.
+    const { separator } = renderGroup('vertical');
+    const variants = ariaVariantsOn(separator);
+
+    expect(variants.length).toBeGreaterThan(0);
+    for (const [attr, value] of variants) {
+      expect(
+        separator.matches(`[${attr}="${value}"]`),
+        `dead selector: no [${attr}="${value}"] on the rendered separator`,
+      ).toBe(true);
+    }
+  });
+
+  it('documents why the wrapper exists: v4 emits no panel-group-direction', () => {
+    // If this ever fails, the library started emitting the v3 attribute again
+    // and `ui/resizable.tsx`'s own variants would carry the stacked case — at
+    // which point this wrapper is redundant rather than load-bearing.
+    const { container } = renderGroup('vertical');
+    expect(container.querySelectorAll('[data-panel-group-direction]')).toHaveLength(0);
+  });
+});
diff --git a/packages/components/src/custom/index.ts b/packages/components/src/custom/index.ts
index 3db4a40f8..1e1438985 100644
--- a/packages/components/src/custom/index.ts
+++ b/packages/components/src/custom/index.ts
@@ -12,6 +12,7 @@ export * from './item';
 export * from './kbd';
 export * from './native-select';
 export * from './navigation-overlay';
+export * from './resizable';
 export * from './section-header';
 export * from './spinner';
 export * from './sort-builder';
diff --git a/packages/components/src/custom/navigation-overlay.tsx b/packages/components/src/custom/navigation-overlay.tsx
index 971bc9159..5860463f1 100644
--- a/packages/components/src/custom/navigation-overlay.tsx
+++ b/packages/components/src/custom/navigation-overlay.tsx
@@ -64,7 +64,7 @@ import {
   ResizablePanelGroup,
   ResizablePanel,
   ResizableHandle,
-} from '../ui/resizable';
+} from './resizable';
 import { usePopperAwareInteractOutside } from './mobile-dialog-content';
 
 /** Navigation mode type — matches ViewNavigationConfig.mode */
diff --git a/packages/components/src/custom/resizable.tsx b/packages/components/src/custom/resizable.tsx
new file mode 100644
index 000000000..60c9436dd
--- /dev/null
+++ b/packages/components/src/custom/resizable.tsx
@@ -0,0 +1,85 @@
+/**
+ * ObjectUI
+ * Copyright (c) 2024-present ObjectStack Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use client';
+
+import * as React from 'react';
+
+import { cn } from '../lib/utils';
+import {
+  ResizableHandle as ShadcnResizableHandle,
+  ResizablePanel,
+  ResizablePanelGroup,
+} from '../ui/resizable';
+
+/**
+ * Orientation-correct `ResizableHandle`.
+ *
+ * `ui/resizable.tsx` (a no-touch Shadcn-sync file, AGENTS.md #7) is from the
+ * `react-resizable-panels` **v3** generation: every one of its stacked-group
+ * styles is keyed on `data-[panel-group-direction=vertical]`. v4 — the
+ * installed major — never emits that attribute. The only data attributes it
+ * puts in the DOM are `data-group`, `data-panel`, `data-separator`,
+ * `data-disabled` and `data-testid`, so all seven variants plus the grip's
+ * `[&[data-panel-group-direction=vertical]>div]:rotate-90` are dead selectors.
+ *
+ * The visible consequence: in a **stacked** group the divider kept the base
+ * `w-px` and rendered as a 1px-wide, 16px-tall sliver (its whole height came
+ * from the grip) instead of a full-width 1px-tall rule, and the grip never
+ * rotated. Measured in a browser before this wrapper existed: 1×16px, with a
+ * 4×16px `::after` hit area pinned to the group's left edge.
+ *
+ * What v4 *does* emit on the separator is `aria-orientation`, so that is what
+ * these variants key on — matching upstream Shadcn, which has since been
+ * regenerated for v4 the same way.
+ *
+ * ⚠️ The attribute value is the **inverse** of the group's `orientation`, and
+ * deliberately so: `aria-orientation` describes the separator, not the group.
+ * A `orientation="vertical"` group stacks its panels, so the divider between
+ * them is a *horizontal* line — hence `aria-orientation="horizontal"`.
+ *
+ *   group orientation="horizontal" → panels side-by-side → separator aria-orientation="vertical"   (base classes)
+ *   group orientation="vertical"   → panels stacked      → separator aria-orientation="horizontal" (these classes)
+ *
+ * Each variant below is the live counterpart of a dead `data-[…]` one still
+ * carried by the base class string. They win on specificity: an attribute
+ * selector scores above the bare utility class it overrides, which is the same
+ * mechanism upstream relies on within its single file.
+ */
+const HORIZONTAL_SEPARATOR_CLASSES = [
+  // The rule itself: a full-width hairline instead of a 1px-wide sliver.
+  'aria-[orientation=horizontal]:h-px',
+  'aria-[orientation=horizontal]:w-full',
+  // The `::after` drag hit area: a 4px strip along the rule, not across it.
+  'aria-[orientation=horizontal]:after:left-0',
+  'aria-[orientation=horizontal]:after:h-1',
+  'aria-[orientation=horizontal]:after:w-full',
+  'aria-[orientation=horizontal]:after:translate-x-0',
+  'aria-[orientation=horizontal]:after:-translate-y-1/2',
+  // The grip glyph reads as a drag affordance only when it crosses the rule.
+  '[&[aria-orientation=horizontal]>div]:rotate-90',
+].join(' ');
+
+export type ResizableHandleProps = React.ComponentProps<typeof ShadcnResizableHandle>;
+
+const ResizableHandle = ({ className, ...props }: ResizableHandleProps) => (
+  <ShadcnResizableHandle className={cn(HORIZONTAL_SEPARATOR_CLASSES, className)} {...props} />
+);
+
+/**
+ * `ResizablePanelGroup` and `ResizablePanel` need no wrapping and are
+ * re-exported as-is: the group's `flex-direction` comes from an inline style
+ * the library writes itself (`row` / `column` from `orientation`), so its own
+ * dead `data-[panel-group-direction=vertical]:flex-col` is inert but harmless.
+ * They pass through here purely so callers have one import site for the trio.
+ *
+ * These are the *only* public `Resizable*` exports — `ui/index.ts` deliberately
+ * does not re-export `./resizable`, so no consumer can reach the unpatched
+ * handle through `@object-ui/components` and silently lose the stacked case.
+ */
+export { ResizableHandle, ResizablePanel, ResizablePanelGroup };
diff --git a/packages/components/src/renderers/complex/resizable.tsx b/packages/components/src/renderers/complex/resizable.tsx
index 626fd0cb5..ab16d8024 100644
--- a/packages/components/src/renderers/complex/resizable.tsx
+++ b/packages/components/src/renderers/complex/resizable.tsx
@@ -9,11 +9,11 @@
 import React from 'react';
 import { ComponentRegistry } from '@object-ui/core';
 import type { ResizableSchema } from '@object-ui/types';
-import { 
-  ResizablePanelGroup, 
-  ResizablePanel, 
-  ResizableHandle 
-} from '../../ui';
+import {
+  ResizablePanelGroup,
+  ResizablePanel,
+  ResizableHandle
+} from '../../custom/resizable';
 import { renderChildren } from '../../lib/utils';
 
 ComponentRegistry.register('resizable', 
diff --git a/packages/components/src/renderers/form/__tests__/form-field-panes.test.tsx b/packages/components/src/renderers/form/__tests__/form-field-panes.test.tsx
index f7db5d830..61bcff558 100644
--- a/packages/components/src/renderers/form/__tests__/form-field-panes.test.tsx
+++ b/packages/components/src/renderers/form/__tests__/form-field-panes.test.tsx
@@ -214,4 +214,24 @@ describe('FormSchema.fieldPanes — one form across the split (#2153)', () => {
     const separator = container.querySelector('[role="separator"]')!;
     expect(separator.getAttribute('aria-disabled')).not.toBe('true');
   });
+
+  it('gives a vertical split a full-width divider, not a 1px sliver', () => {
+    // `fieldPanesOrientation: 'vertical'` stacks the panes, so the divider is a
+    // horizontal rule. It used to render as a 1px-wide vertical sliver: the
+    // Shadcn handle styled that case with `data-[panel-group-direction=vertical]`
+    // variants, which react-resizable-panels v4 never triggers. The form renders
+    // through `custom/resizable`, which re-keys them onto the separator's own
+    // `aria-orientation` — see custom/resizable.tsx and
+    // __tests__/resizable-orientation.test.tsx.
+    const { container } = renderSplitForm({ fieldPanesOrientation: 'vertical' });
+    const separator = container.querySelector('[role="separator"]')!;
+
+    expect(separator.getAttribute('aria-orientation')).toBe('horizontal');
+    expect([...separator.classList]).toContain('aria-[orientation=horizontal]:w-full');
+
+    // …and the default (side-by-side) split still gets the vertical divider.
+    cleanup();
+    const sideBySide = renderSplitForm().container.querySelector('[role="separator"]')!;
+    expect(sideBySide.getAttribute('aria-orientation')).toBe('vertical');
+  });
 });
diff --git a/packages/components/src/renderers/form/form.tsx b/packages/components/src/renderers/form/form.tsx
index a469c19b7..58b266ce8 100644
--- a/packages/components/src/renderers/form/form.tsx
+++ b/packages/components/src/renderers/form/form.tsx
@@ -24,7 +24,7 @@ import {
 } from '../../ui/select';
 import { renderChildren } from '../../lib/utils';
 import { Tabs, TabsContent, TabsList, TabsTrigger } from '../../ui/tabs';
-import { ResizablePanelGroup, ResizablePanel, ResizableHandle } from '../../ui/resizable';
+import { ResizablePanelGroup, ResizablePanel, ResizableHandle } from '../../custom/resizable';
 import { Alert, AlertDescription } from '../../ui/alert';
 import { toast } from '../../ui/sonner';
 import { AlertCircle, ChevronDown, ChevronRight, Loader2, Maximize2, Check, X } from 'lucide-react';
diff --git a/packages/components/src/ui/index.ts b/packages/components/src/ui/index.ts
index ef900fb3b..e77bc2016 100644
--- a/packages/components/src/ui/index.ts
+++ b/packages/components/src/ui/index.ts
@@ -36,7 +36,11 @@ export * from './pagination';
 export * from './popover';
 export * from './progress';
 export * from './radio-group';
-export * from './resizable';
+// NOT re-exported: `./resizable` ships v3-era `data-[panel-group-direction]`
+// styles that react-resizable-panels v4 never triggers, so its handle has no
+// stacked-group appearance. `custom/resizable.tsx` wraps it and owns the public
+// `Resizable*` names — exporting both here would also collide (TS2308), since
+// `src/index.ts` star-exports `./ui` and `./custom` side by side.
 export * from './scroll-area';
 export * from './select';
 export * from './separator';