diff --git a/.ai/guidelines/craft.md b/.ai/guidelines/craft.md
index 74e578d3ab6..c701e9c2146 100644
--- a/.ai/guidelines/craft.md
+++ b/.ai/guidelines/craft.md
@@ -39,3 +39,7 @@ Some files contain Unicode characters in comments and strings. If a text edit fa
 - Prefix the title with the origin version branch, such as `[6.x]` or `[5.x]`.
 - Use a `### Description` section and add `### Related issues` only when applicable. Do not add validation summaries.
 - When the branch already identifies a Linear issue, reference the related GitHub issue instead of repeating the Linear identifier.
+
+## Regression tests
+
+- After a bug fix or behavior change is verified working, propose adding a test that covers it — don't leave that as an unprompted afterthought.
diff --git a/AGENTS.md b/AGENTS.md
index 4f30145ccc1..609e2aaa694 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -48,6 +48,10 @@ Some files contain Unicode characters in comments and strings. If a text edit fa
 
 - When the branch already identifies a Linear issue, reference the related GitHub issue instead of repeating the Linear identifier.
 
+## Regression tests
+
+- After a bug fix or behavior change is verified working, propose adding a test that covers it — don't leave that as an unprompted afterthought.
+
 === foundation rules ===
 
 # Laravel Boost Guidelines
diff --git a/resources/js/common/components/CpSidebar.test.ts b/resources/js/common/components/CpSidebar.test.ts
new file mode 100644
index 00000000000..887f59327e8
--- /dev/null
+++ b/resources/js/common/components/CpSidebar.test.ts
@@ -0,0 +1,124 @@
+import {afterEach, expect, it, vi} from 'vite-plus/test';
+import {computed, createApp, nextTick, reactive, ref, type App} from 'vue';
+
+const state = vi.hoisted(() => ({
+  page: null as any,
+  sidebar: null as unknown as {
+    mode: 'docked' | 'floating';
+    visibility: 'visible' | 'hidden';
+  },
+  toggle: vi.fn(),
+}));
+state.sidebar = reactive({mode: 'docked', visibility: 'visible'});
+
+vi.mock('@inertiajs/vue3', async () => ({
+  ...(await vi.importActual('@inertiajs/vue3')),
+  usePage: () => state.page,
+}));
+
+vi.mock('@/common/composables/useGlobalSidebar', () => ({
+  useGlobalSidebar: () => ({
+    sidebar: state.sidebar,
+    collapsed: computed(
+      () =>
+        state.sidebar.mode === 'docked' && state.sidebar.visibility === 'hidden'
+    ),
+    toggle: state.toggle,
+    icon: computed(() => 'arrow-left-to-line'),
+  }),
+}));
+
+// Below `lg` the sidebar renders its header, with the close toggle the floating
+// focus test expects to land on.
+vi.mock('@/common/composables/useCpBreakpoints', () => ({
+  cpBreakpoints: {greaterOrEqual: () => ref(false)},
+}));
+
+let app: App | null = null;
+const extraElements: HTMLElement[] = [];
+
+afterEach(() => {
+  app?.unmount();
+  app = null;
+  extraElements.splice(0).forEach((el) => el.remove());
+});
+
+function appendElement<T extends HTMLElement>(el: T): T {
+  document.body.append(el);
+  extraElements.push(el);
+  return el;
+}
+
+async function mountSidebar(
+  mode: 'docked' | 'floating',
+  visibility: 'visible' | 'hidden'
+) {
+  state.sidebar = reactive({mode, visibility});
+  state.toggle = vi.fn(() => {
+    state.sidebar.visibility =
+      state.sidebar.visibility === 'visible' ? 'hidden' : 'visible';
+  });
+  state.page = reactive({
+    url: '/admin',
+    props: {
+      craft: {
+        maintenanceMode: false,
+        system: {name: 'Craft CMS', icon: null},
+        site: null,
+        app: {version: '6.0.0', edition: {name: 'Pro'}},
+        general: {cpTrigger: 'admin'},
+        nav: [],
+        navBadges: {},
+      },
+      queue: {
+        displayedJob: null,
+        hasReservedJobs: false,
+        hasWaitingJobs: false,
+      },
+    },
+  });
+
+  const {default: CpSidebar} = await import('./CpSidebar.vue');
+  const container = appendElement(document.createElement('div'));
+  app = createApp(CpSidebar);
+
+  app.mount(container);
+  await nextTick();
+
+  return container;
+}
+
+it('keeps focus on the collapse item when the docked sidebar expands', async () => {
+  const container = await mountSidebar('docked', 'hidden');
+  const collapseItem = () =>
+    container.querySelector<HTMLElement>('.cp-sidebar__footer craft-nav-item')!;
+
+  collapseItem().click();
+
+  expect(state.sidebar.visibility).toBe('visible');
+  await vi.waitFor(() => expect(document.activeElement).toBe(collapseItem()));
+});
+
+it('keeps focus on the collapse item when the docked sidebar collapses', async () => {
+  const container = await mountSidebar('docked', 'visible');
+  const collapseItem = () =>
+    container.querySelector<HTMLElement>('.cp-sidebar__footer craft-nav-item')!;
+
+  collapseItem().click();
+
+  expect(state.sidebar.visibility).toBe('hidden');
+  await vi.waitFor(() => expect(document.activeElement).toBe(collapseItem()));
+});
+
+it('moves focus into the sidebar when it becomes visible while focus was on an external control', async () => {
+  const container = await mountSidebar('floating', 'hidden');
+  const externalControl = appendElement(document.createElement('button'));
+  externalControl.focus();
+
+  state.sidebar.visibility = 'visible';
+  await vi.waitFor(() =>
+    expect(document.activeElement).toBe(
+      container.querySelector('#sidebar-toggle')
+    )
+  );
+});