diff --git a/web/src/shell/AppShell.test.tsx b/web/src/shell/AppShell.test.tsx
index 539231c..a7ba038 100644
--- a/web/src/shell/AppShell.test.tsx
+++ b/web/src/shell/AppShell.test.tsx
@@ -2,7 +2,7 @@ import { render, screen, within } from '@testing-library/react'
 import { MemoryRouter } from 'react-router'
 import { afterEach, describe, expect, it } from 'vitest'
 import App from '../App.tsx'
-import { declaredStyle } from '../test/cssRules.ts'
+import { declaredStyle, outerSize } from '../test/cssRules.ts'
 import { expectMinHitArea } from '../test/hitArea.ts'
 import { mockViewport } from '../test/matchMedia.ts'
 import bottomBarStyles from './BottomBar.module.css'
@@ -69,6 +69,7 @@ describe('AppShell', () => {
     expect(bar.right).toBe('12px')
     expect(bar.bottom).toBe('calc(16px + env(safe-area-inset-bottom))')
     expect(bar.height).toBe('62px')
+    expect(outerSize(nav(), 'height')).toBe(62)
     expect(bar.background).toBe('var(--color-text)')
     expect(bar.borderRadius).toBe('var(--radius-pill)')
   })
@@ -111,9 +112,12 @@ describe('AppShell', () => {
     expect(screen.getAllByRole('navigation')).toHaveLength(1)
     expect(nav()).toHaveClass(railStyles.rail!)
     expect(declaredStyle(nav()).width).toBe('96px')
+    expect(outerSize(nav(), 'width')).toBe(96)
     expect(declaredStyle(nav()).background).toBe('var(--color-surface)')
     const mark = within(nav()).getByText('M')
     expect(declaredStyle(mark).width).toBe('48px')
+    expect(outerSize(mark, 'width')).toBe(48)
+    expect(outerSize(mark, 'height')).toBe(48)
     expect(declaredStyle(mark).background).toBe('var(--color-accent)')
     expect(screen.queryByText('MealQuest')).not.toBeInTheDocument()
     expect(screen.queryByText('Our kitchen')).not.toBeInTheDocument()
@@ -122,6 +126,8 @@ describe('AppShell', () => {
       const pill = link.firstElementChild!
       expect(declaredStyle(pill).width).toBe('56px')
       expect(declaredStyle(pill).height).toBe('36px')
+      expect(outerSize(pill, 'width')).toBe(56)
+      expect(outerSize(pill, 'height')).toBe(36)
       expect(declaredStyle(link).fontSize).toBe('11px')
       expect(declaredStyle(link).fontWeight).toBe('700')
       expectMinHitArea(link)
@@ -138,6 +144,8 @@ describe('AppShell', () => {
     expect(declaredStyle(sidebar).width).toBe('208px')
     expect(declaredStyle(sidebar).background).toBe('var(--color-surface)')
     expect(declaredStyle(sidebar).padding).toBe('28px 16px')
+    // The 16 px side padding sits inside the 208 px, not outside it.
+    expect(outerSize(sidebar, 'width')).toBe(208)
 
     const wordmark = within(sidebar).getByText('MealQuest')
     expect(declaredStyle(wordmark).fontFamily).toBe('var(--font-heading)')
diff --git a/web/src/shell/BottomBar.module.css b/web/src/shell/BottomBar.module.css
index 92d8cb7..6d4e044 100644
--- a/web/src/shell/BottomBar.module.css
+++ b/web/src/shell/BottomBar.module.css
@@ -5,6 +5,7 @@
   bottom: calc(16px + env(safe-area-inset-bottom));
   height: 62px;
   padding: 0 6px;
+  box-sizing: border-box;
   display: flex;
   justify-content: space-around;
   align-items: center;
diff --git a/web/src/shell/Rail.module.css b/web/src/shell/Rail.module.css
index d84eefd..f9da188 100644
--- a/web/src/shell/Rail.module.css
+++ b/web/src/shell/Rail.module.css
@@ -1,6 +1,7 @@
 .rail {
   width: 96px;
   padding: 24px 0;
+  box-sizing: border-box;
   display: flex;
   flex-direction: column;
   align-items: center;
diff --git a/web/src/shell/Sidebar.module.css b/web/src/shell/Sidebar.module.css
index 05f223c..4aa11ce 100644
--- a/web/src/shell/Sidebar.module.css
+++ b/web/src/shell/Sidebar.module.css
@@ -1,6 +1,7 @@
 .sidebar {
   width: 208px;
   padding: 28px 16px;
+  box-sizing: border-box;
   display: flex;
   flex-direction: column;
   background: var(--color-surface);
diff --git a/web/src/test/cssRules.ts b/web/src/test/cssRules.ts
index e8413aa..3260596 100644
--- a/web/src/test/cssRules.ts
+++ b/web/src/test/cssRules.ts
@@ -48,3 +48,28 @@ export function animationKeyframes(element: Element): CSSKeyframesRule {
   if (!keyframes) throw new Error(`no @keyframes named ${JSON.stringify(name)}`)
   return keyframes
 }
+
+function px(value: string, what: string): number {
+  if (value === '') return 0
+  const match = /^(\d+(?:\.\d+)?)px$/.exec(value)
+  if (!match) throw new Error(`${what} ${JSON.stringify(value)} is not a px length`)
+  return Number(match[1])
+}
+
+// The border-box size the element's declared rules give it on one axis:
+// width or height, plus padding and border unless box-sizing is border-box.
+// jsdom does no layout (getBoundingClientRect is all zeros), so this is the
+// way to catch padding that would push a fixed box past its specified size.
+export function outerSize(element: Element, axis: 'width' | 'height'): number {
+  const style = declaredStyle(element)
+  const size = px(style.getPropertyValue(axis), axis)
+  if (style.boxSizing === 'border-box') return size
+  const sides = axis === 'width' ? ['left', 'right'] : ['top', 'bottom']
+  return sides.reduce(
+    (total, side) =>
+      total +
+      px(style.getPropertyValue(`padding-${side}`), `padding-${side}`) +
+      px(style.getPropertyValue(`border-${side}-width`), `border-${side}-width`),
+    size,
+  )
+}