alphagov · colinrotherham · Dec 15, 2023 · Dec 13, 2023 · Dec 13, 2023 · Dec 13, 2023
diff --git a/packages/govuk-frontend/src/govuk/common/index.mjs b/packages/govuk-frontend/src/govuk/common/index.mjs
@@ -133,6 +133,27 @@ export function getFragmentFromUrl(url) {
   return url.split('#').pop()
 }
 
+/**
+ * Get GOV.UK Frontend breakpoint value from CSS custom property
+ *
+ * @private
+ * @param {string} name - Breakpoint name
+ * @returns {{ property: string, value?: string }} Breakpoint object
+ */
+export function getBreakpoint(name) {
+  const property = `--govuk-frontend-breakpoint-${name}`
+
+  // Get value from `<html>` with breakpoints on CSS :root
+  const value = window
+    .getComputedStyle(document.documentElement)
+    .getPropertyValue(property)
+
+  return {
+    property,
+    value: value || undefined
+  }
+}
+
 /**
  * Move focus to element
  *

diff --git a/packages/govuk-frontend/src/govuk/components/header/header.mjs b/packages/govuk-frontend/src/govuk/components/header/header.mjs
@@ -1,3 +1,4 @@
+import { getBreakpoint } from '../../common/index.mjs'
 import { ElementError } from '../../errors/index.mjs'
 import { GOVUKFrontendComponent } from '../../govuk-frontend-component.mjs'
 
@@ -83,23 +84,42 @@ export class Header extends GOVUKFrontendComponent {
     this.$menu = $menu
     this.$menuButton = $menuButton
 
-    // Set the matchMedia to the govuk-frontend desktop breakpoint
-    this.mql = window.matchMedia('(min-width: 48.0625em)')
+    this.setupResponsiveChecks()
+
+    this.$menuButton.addEventListener('click', () =>
+      this.handleMenuButtonClick()
+    )
+  }
+
+  /**
+   * Setup viewport resize check
+   *
+   * @private
+   */
+  setupResponsiveChecks() {
+    const breakpoint = getBreakpoint('desktop')
+
+    if (!breakpoint.value) {
+      throw new ElementError({
+        componentName: 'Header',
+        identifier: `CSS custom property (\`${breakpoint.property}\`) on pseudo-class \`:root\``
+      })
+    }
+
+    // Media query list for GOV.UK Frontend desktop breakpoint
+    this.mql = window.matchMedia(`(min-width: ${breakpoint.value})`)
 
     // MediaQueryList.addEventListener isn't supported by Safari < 14 so we need
     // to be able to fall back to the deprecated MediaQueryList.addListener
     if ('addEventListener' in this.mql) {
-      this.mql.addEventListener('change', () => this.syncState())
+      this.mql.addEventListener('change', () => this.checkMode())
     } else {
       // @ts-expect-error Property 'addListener' does not exist
       // eslint-disable-next-line @typescript-eslint/no-unsafe-call
-      this.mql.addListener(() => this.syncState())
+      this.mql.addListener(() => this.checkMode())
     }
 
-    this.syncState()
-    this.$menuButton.addEventListener('click', () =>
-      this.handleMenuButtonClick()
-    )
+    this.checkMode()
   }
 
   /**
@@ -112,7 +132,7 @@ export class Header extends GOVUKFrontendComponent {
    *
    * @private
    */
-  syncState() {
+  checkMode() {
     if (!this.mql || !this.$menu || !this.$menuButton) {
       return
     }
@@ -142,7 +162,7 @@ export class Header extends GOVUKFrontendComponent {
    */
   handleMenuButtonClick() {
     this.menuIsOpen = !this.menuIsOpen
-    this.syncState()
+    this.checkMode()
   }
 
   /**

diff --git a/packages/govuk-frontend/src/govuk/components/tabs/tabs.mjs b/packages/govuk-frontend/src/govuk/components/tabs/tabs.mjs
@@ -1,4 +1,4 @@
-import { getFragmentFromUrl } from '../../common/index.mjs'
+import { getBreakpoint, getFragmentFromUrl } from '../../common/index.mjs'
 import { ElementError } from '../../errors/index.mjs'
 import { GOVUKFrontendComponent } from '../../govuk-frontend-component.mjs'
 
@@ -105,7 +105,17 @@ export class Tabs extends GOVUKFrontendComponent {
    * @private
    */
   setupResponsiveChecks() {
-    this.mql = window.matchMedia('(min-width: 40.0625em)')
+    const breakpoint = getBreakpoint('tablet')
+
+    if (!breakpoint.value) {
+      throw new ElementError({
+        componentName: 'Tabs',
+        identifier: `CSS custom property (\`${breakpoint.property}\`) on pseudo-class \`:root\``
+      })
+    }
+
+    // Media query list for GOV.UK Frontend tablet breakpoint
+    this.mql = window.matchMedia(`(min-width: ${breakpoint.value})`)
 
     // MediaQueryList.addEventListener isn't supported by Safari < 14 so we need
     // to be able to fall back to the deprecated MediaQueryList.addListener

diff --git a/packages/govuk-frontend/src/govuk/core/_all.scss b/packages/govuk-frontend/src/govuk/core/_all.scss
@@ -1,4 +1,4 @@
-@import "govuk-frontend-version";
+@import "govuk-frontend-properties";
 @import "links";
 @import "lists";
 @import "typography";

diff --git a/...c/govuk/core/_govuk-frontend-version.scss → ...ovuk/core/_govuk-frontend-properties.scss b/...c/govuk/core/_govuk-frontend-version.scss → ...ovuk/core/_govuk-frontend-properties.scss
@@ -2,4 +2,9 @@
   // This variable is automatically overwritten during builds and releases.
   // It doesn't need to be updated manually.
   --govuk-frontend-version: "development";
+
+  // CSS custom property for each breakpoint
+  @each $name, $value in $govuk-breakpoints {
+    --govuk-frontend-breakpoint-#{$name}: #{govuk-px-to-rem($value)};
+  }
 }
diff --git a/packages/govuk-frontend/tasks/build/release.unit.test.mjs b/packages/govuk-frontend/tasks/build/release.unit.test.mjs
@@ -49,8 +49,18 @@ describe('dist/', () => {
       )
     })
 
-    it('should contain version number custom property', () => {
+    it('should contain CSS custom properties', () => {
+      // GOV.UK Frontend version number
       expect(stylesheet).toContain(`--govuk-frontend-version:"${pkg.version}"`)
+
+      // Breakpoints for `window.matchMedia()` in Header, Tabs components
+      expect(stylesheet).toContain('--govuk-frontend-breakpoint-mobile:20rem')
+      expect(stylesheet).toContain(
+        '--govuk-frontend-breakpoint-tablet:40.0625rem'
+      )
+      expect(stylesheet).toContain(
+        '--govuk-frontend-breakpoint-desktop:48.0625rem'
+      )
     })
   })
 
@@ -70,7 +80,7 @@ describe('dist/', () => {
         '../packages/govuk-frontend/src/govuk/all.scss'
       )
       expect(sourcemap.sources).toContain(
-        '../packages/govuk-frontend/src/govuk/core/_govuk-frontend-version.scss'
+        '../packages/govuk-frontend/src/govuk/core/_govuk-frontend-properties.scss'
       )
     })
   })