diff --git a/src/lib/__tests__/use-layout.test.ts b/src/lib/__tests__/use-layout.test.ts
new file mode 100644
index 00000000..13746b91
--- /dev/null
+++ b/src/lib/__tests__/use-layout.test.ts
@@ -0,0 +1,52 @@
+import { describe, it, expect, vi } from 'vitest';
+
+vi.mock('react-native', () => ({ useWindowDimensions: () => ({ width: 411, height: 914 }) }));
+
+import { layoutFor, listPaneWidth } from '../use-layout';
+
+describe('layoutFor', () => {
+  it('keeps a phone in portrait on the single-pane layout', () => {
+    expect(layoutFor(411, 914)).toEqual({
+      widthClass: 'compact', shortHeight: false, isSplit: false, hasSidebar: false,
+    });
+  });
+
+  it('keeps a phone in landscape single-pane although it is wide', () => {
+    const l = layoutFor(914, 411);
+    expect(l.widthClass).toBe('expanded');
+    expect(l.shortHeight).toBe(true);
+    expect(l.isSplit).toBe(false);
+    expect(l.hasSidebar).toBe(false);
+  });
+
+  it('splits a tablet in portrait without a sidebar', () => {
+    expect(layoutFor(800, 1280)).toEqual({
+      widthClass: 'medium', shortHeight: false, isSplit: true, hasSidebar: false,
+    });
+  });
+
+  it('splits a tablet in landscape and adds the sidebar', () => {
+    expect(layoutFor(1280, 800)).toEqual({
+      widthClass: 'expanded', shortHeight: false, isSplit: true, hasSidebar: true,
+    });
+  });
+
+  it('uses the window, so a narrow split-screen window is compact', () => {
+    expect(layoutFor(500, 800).isSplit).toBe(false);
+  });
+
+  it('puts the breakpoints at 600 and 840', () => {
+    expect(layoutFor(599, 900).widthClass).toBe('compact');
+    expect(layoutFor(600, 900).widthClass).toBe('medium');
+    expect(layoutFor(839, 900).widthClass).toBe('medium');
+    expect(layoutFor(840, 900).widthClass).toBe('expanded');
+  });
+});
+
+describe('listPaneWidth', () => {
+  it('stays between 320 and 420 dp', () => {
+    expect(listPaneWidth(600)).toBe(320);
+    expect(listPaneWidth(1000)).toBe(340);
+    expect(listPaneWidth(2000)).toBe(420);
+  });
+});
diff --git a/src/lib/use-layout.ts b/src/lib/use-layout.ts
new file mode 100644
index 00000000..eb77de83
--- /dev/null
+++ b/src/lib/use-layout.ts
@@ -0,0 +1,47 @@
+import { useWindowDimensions } from 'react-native';
+
+/**
+ * Window size classes for the adaptive layouts (docs/ux-audit-2026-10.md,
+ * "Adaptive layout plan"). They come from the window, not the device, so
+ * split-screen, freeform windows and foldables get the layout that fits.
+ */
+export type WidthClass = 'compact' | 'medium' | 'expanded';
+
+export const MEDIUM_MIN_WIDTH = 600;
+export const EXPANDED_MIN_WIDTH = 840;
+/** Below this a window is "short": a phone in landscape. */
+export const SHORT_MAX_HEIGHT = 480;
+
+export interface Layout {
+  widthClass: WidthClass;
+  /** A phone in landscape: compress chrome, keep the single-pane layout. */
+  shortHeight: boolean;
+  /** List and detail side by side (medium or expanded, and not short). */
+  isSplit: boolean;
+  /** A persistent sidebar next to the list (expanded and not short). */
+  hasSidebar: boolean;
+}
+
+export function layoutFor(width: number, height: number): Layout {
+  const widthClass: WidthClass =
+    width >= EXPANDED_MIN_WIDTH ? 'expanded' : width >= MEDIUM_MIN_WIDTH ? 'medium' : 'compact';
+  const shortHeight = height < SHORT_MAX_HEIGHT;
+  const isSplit = widthClass !== 'compact' && !shortHeight;
+  return { widthClass, shortHeight, isSplit, hasSidebar: isSplit && widthClass === 'expanded' };
+}
+
+export function useLayout(): Layout {
+  const { width, height } = useWindowDimensions();
+  return layoutFor(width, height);
+}
+
+/**
+ * Width of the list pane next to a detail pane: a third of the window,
+ * between 320 and 420 dp (the webmail's list column is about 400).
+ */
+export function listPaneWidth(windowWidth: number): number {
+  return Math.round(Math.max(320, Math.min(420, windowWidth * 0.34)));
+}
+
+/** Width for sheets, dialogs and floating cards that would span a wide window. */
+export const SHEET_MAX_WIDTH = 560;