diff --git a/docs/qa.md b/docs/qa.md
index ee124d6c..8dc58351 100644
--- a/docs/qa.md
+++ b/docs/qa.md
@@ -4,6 +4,12 @@ Run this checklist before publishing releases or merging changes that touch uplo
 
 Before starting, open `docs/kitchen-sink.html` in a browser to review the modal surface reference. With that page as your visual baseline, perform this quick accessibility spot check in the running app:
 
+Automated regression coverage now exercises these flows across both the legacy and design-system variants:
+
+* `tests/video-modal-accessibility.test.mjs` validates sticky header transitions, scroll locking, and zap popover accessibility for the video modal.
+* `tests/modal-accessibility.test.mjs` mirrors upload/edit modal accessibility along with their form validation and mode toggles.
+* `tests/profile-modal-controller.test.mjs` verifies profile modal focus restoration and navigation button state changes.
+
 * Launch the site, trigger any video card or thumbnail to open the playback modal, and keep track of the element you used to open it.
 * Scroll the modal content down and back up; confirm the sticky navigation bar hides on downward scroll and reappears when you reverse direction.
 * Activate the zap button to display the popover and tab through its fields to confirm focus stays within the dialog; press `Escape` to close it and ensure focus returns to the zap trigger.
diff --git a/tests/modal-accessibility.test.mjs b/tests/modal-accessibility.test.mjs
index 53efe00e..a70928d0 100644
--- a/tests/modal-accessibility.test.mjs
+++ b/tests/modal-accessibility.test.mjs
@@ -11,6 +11,11 @@ import {
   openStaticModal,
   closeStaticModal,
 } from "../js/ui/components/staticModalAccessibility.js";
+import {
+  setFeatureDesignSystemEnabled,
+  resetRuntimeFlags,
+} from "../js/constants.js";
+import { applyDesignSystemAttributes } from "../js/designSystem.js";
 
 const uploadMarkupPromise = readFile(
   new URL("../components/upload-modal.html", import.meta.url),
@@ -90,101 +95,388 @@ async function flushMicrotasks() {
   await Promise.resolve();
 }
 
-test("UploadModal closes on Escape and restores trigger focus", async () => {
-  const markup = await uploadMarkupPromise;
-  const cleanups = [];
-  cleanups.push(
-    installDom(
-      "<!DOCTYPE html><html><body><button id=\"trigger\">Upload</button><div id=\"modalContainer\"></div></body></html>",
-    ),
-  );
-  cleanups.push(
-    stubFetch(
-      new Map([["components/upload-modal.html", markup]]),
-    ),
-  );
+for (const designSystemEnabled of [false, true]) {
+  const modeLabel = designSystemEnabled ? "design-system" : "legacy";
 
-  const modal = new UploadModal({
-    removeTrackingScripts: () => {},
-    setGlobalModalState: () => {},
-  });
-  await modal.load();
+  test(
+    `[${modeLabel}] UploadModal closes on Escape and restores trigger focus`,
+    async (t) => {
+      const markup = await uploadMarkupPromise;
+      const cleanups = [];
+      cleanups.push(
+        installDom(
+          "<!DOCTYPE html><html><body><button id=\"trigger\">Upload</button><div id=\"modalContainer\"></div></body></html>",
+        ),
+      );
+      cleanups.push(
+        stubFetch(new Map([["components/upload-modal.html", markup]])),
+      );
 
-  const trigger = document.getElementById("trigger");
-  assert.ok(trigger);
+      setFeatureDesignSystemEnabled(designSystemEnabled);
 
-  modal.open({ triggerElement: trigger });
-  await flushMicrotasks();
+      const modal = new UploadModal({
+        removeTrackingScripts: () => {},
+        setGlobalModalState: () => {},
+      });
+      await modal.load();
+      applyDesignSystemAttributes(document);
 
-  document.dispatchEvent(
-    new window.KeyboardEvent("keydown", { key: "Escape", bubbles: true, cancelable: true }),
-  );
-  await flushMicrotasks();
+      const trigger = document.getElementById("trigger");
+      assert.ok(trigger);
 
-  const root = modal.getRoot();
-  assert.ok(root?.classList.contains("hidden"));
-  assert.strictEqual(document.activeElement, trigger);
+      modal.open({ triggerElement: trigger });
+      await flushMicrotasks();
 
-  modal.destroy();
-  cleanups.reverse().forEach((fn) => fn?.());
-});
+      document.dispatchEvent(
+        new window.KeyboardEvent("keydown", {
+          key: "Escape",
+          bubbles: true,
+          cancelable: true,
+        }),
+      );
+      await flushMicrotasks();
 
-test("EditModal Escape closes and restores trigger focus", async () => {
-  const markup = await editMarkupPromise;
-  const cleanups = [];
-  cleanups.push(
-    installDom(
-      "<!DOCTYPE html><html><body><button id=\"trigger\">Edit</button><div id=\"modalContainer\"></div></body></html>",
-    ),
-  );
-  cleanups.push(
-    stubFetch(
-      new Map([["components/edit-video-modal.html", markup]]),
-    ),
-  );
+      const root = modal.getRoot();
+      assert.ok(root?.classList.contains("hidden"));
+      assert.strictEqual(document.activeElement, trigger);
 
-  const modal = new EditModal({
-    removeTrackingScripts: () => {},
-    setGlobalModalState: () => {},
-    sanitizers: {},
-    escapeHtml: (value) => String(value ?? ""),
-    showError: () => {},
-    getMode: () => "live",
-  });
-  await modal.load();
-
-  const trigger = document.getElementById("trigger");
-  assert.ok(trigger);
-
-  await modal.open(
-    {
-      id: "event1",
-      title: "Example",
-      url: "https://example.com/video.mp4",
-      magnet: "",
-      ws: "",
-      xs: "",
-      enableComments: true,
-      isPrivate: false,
-      isNsfw: false,
-      isForKids: false,
+      t.after(() => {
+        modal.destroy();
+        resetRuntimeFlags();
+        cleanups.reverse().forEach((fn) => fn?.());
+      });
     },
-    { triggerElement: trigger },
   );
-  await flushMicrotasks();
 
-  document.dispatchEvent(
-    new window.KeyboardEvent("keydown", { key: "Escape", bubbles: true, cancelable: true }),
+  test(
+    `[${modeLabel}] UploadModal backdrop click closes and restores trigger focus`,
+    async (t) => {
+      const markup = await uploadMarkupPromise;
+      const cleanups = [];
+      cleanups.push(
+        installDom(
+          "<!DOCTYPE html><html><body><button id=\"trigger\">Upload</button><div id=\"modalContainer\"></div></body></html>",
+        ),
+      );
+      cleanups.push(
+        stubFetch(new Map([["components/upload-modal.html", markup]])),
+      );
+
+      setFeatureDesignSystemEnabled(designSystemEnabled);
+
+      const modal = new UploadModal({
+        removeTrackingScripts: () => {},
+        setGlobalModalState: () => {},
+      });
+      await modal.load();
+      applyDesignSystemAttributes(document);
+
+      const trigger = document.getElementById("trigger");
+      assert.ok(trigger);
+
+      modal.open({ triggerElement: trigger });
+      await flushMicrotasks();
+
+      const backdrop = modal.getRoot()?.querySelector?.("[data-dismiss]");
+      assert.ok(backdrop, "expected upload modal backdrop");
+
+      backdrop.dispatchEvent(
+        new window.MouseEvent("click", { bubbles: true, cancelable: true }),
+      );
+      await flushMicrotasks();
+
+      const root = modal.getRoot();
+      assert.ok(root?.classList.contains("hidden"));
+      assert.strictEqual(document.activeElement, trigger);
+
+      t.after(() => {
+        modal.destroy();
+        resetRuntimeFlags();
+        cleanups.reverse().forEach((fn) => fn?.());
+      });
+    },
   );
-  await flushMicrotasks();
 
-  const root = modal.getRoot();
-  assert.ok(root?.classList.contains("hidden"));
-  assert.strictEqual(document.activeElement, trigger);
+  test(
+    `[${modeLabel}] UploadModal mode toggle updates button states`,
+    async (t) => {
+      const markup = await uploadMarkupPromise;
+      const cleanups = [];
+      cleanups.push(
+        installDom(
+          "<!DOCTYPE html><html><body><div id=\"modalContainer\"></div></body></html>",
+        ),
+      );
+      cleanups.push(
+        stubFetch(new Map([["components/upload-modal.html", markup]])),
+      );
 
-  modal.destroy();
-  cleanups.reverse().forEach((fn) => fn?.());
-});
+      setFeatureDesignSystemEnabled(designSystemEnabled);
+
+      const modal = new UploadModal({
+        removeTrackingScripts: () => {},
+        setGlobalModalState: () => {},
+      });
+      await modal.load();
+      applyDesignSystemAttributes(document);
+
+      const customButton = document.querySelector(
+        ".upload-mode-toggle[data-upload-mode=\"custom\"]",
+      );
+      const cloudflareButton = document.querySelector(
+        ".upload-mode-toggle[data-upload-mode=\"cloudflare\"]",
+      );
+      const customSection = document.getElementById("customUploadSection");
+      const cloudflareSection = document.getElementById("cloudflareUploadSection");
+
+      assert.ok(customButton && cloudflareButton);
+      assert.ok(customSection && cloudflareSection);
+
+      modal.setMode("cloudflare");
+
+      assert.equal(cloudflareButton.getAttribute("aria-pressed"), "true");
+      assert.equal(customButton.getAttribute("aria-pressed"), "false");
+      assert.equal(cloudflareSection.classList.contains("hidden"), false);
+      assert.equal(customSection.classList.contains("hidden"), true);
+
+      modal.setMode("custom");
+
+      assert.equal(customButton.getAttribute("aria-pressed"), "true");
+      assert.equal(cloudflareButton.getAttribute("aria-pressed"), "false");
+      assert.equal(customSection.classList.contains("hidden"), false);
+      assert.equal(cloudflareSection.classList.contains("hidden"), true);
+
+      t.after(() => {
+        modal.destroy();
+        resetRuntimeFlags();
+        cleanups.reverse().forEach((fn) => fn?.());
+      });
+    },
+  );
+}
+
+for (const designSystemEnabled of [false, true]) {
+  const modeLabel = designSystemEnabled ? "design-system" : "legacy";
+
+  test(
+    `[${modeLabel}] EditModal Escape closes and restores trigger focus`,
+    async (t) => {
+      const markup = await editMarkupPromise;
+      const cleanups = [];
+      cleanups.push(
+        installDom(
+          "<!DOCTYPE html><html><body><button id=\"trigger\">Edit</button><div id=\"modalContainer\"></div></body></html>",
+        ),
+      );
+      cleanups.push(
+        stubFetch(
+          new Map([["components/edit-video-modal.html", markup]]),
+        ),
+      );
+
+      setFeatureDesignSystemEnabled(designSystemEnabled);
+
+      const modal = new EditModal({
+        removeTrackingScripts: () => {},
+        setGlobalModalState: () => {},
+        sanitizers: {},
+        escapeHtml: (value) => String(value ?? ""),
+        showError: () => {},
+        getMode: () => "live",
+      });
+      await modal.load();
+      applyDesignSystemAttributes(document);
+
+      const trigger = document.getElementById("trigger");
+      assert.ok(trigger);
+
+      await modal.open(
+        {
+          id: "event1",
+          title: "Example",
+          url: "https://example.com/video.mp4",
+          magnet: "",
+          ws: "",
+          xs: "",
+          enableComments: true,
+          isPrivate: false,
+          isNsfw: false,
+          isForKids: false,
+        },
+        { triggerElement: trigger },
+      );
+      await flushMicrotasks();
+
+      document.dispatchEvent(
+        new window.KeyboardEvent("keydown", {
+          key: "Escape",
+          bubbles: true,
+          cancelable: true,
+        }),
+      );
+      await flushMicrotasks();
+
+      const root = modal.getRoot();
+      assert.ok(root?.classList.contains("hidden"));
+      assert.strictEqual(document.activeElement, trigger);
+
+      t.after(() => {
+        modal.destroy();
+        resetRuntimeFlags();
+        cleanups.reverse().forEach((fn) => fn?.());
+      });
+    },
+  );
+
+  test(
+    `[${modeLabel}] EditModal backdrop click closes and restores trigger focus`,
+    async (t) => {
+      const markup = await editMarkupPromise;
+      const cleanups = [];
+      cleanups.push(
+        installDom(
+          "<!DOCTYPE html><html><body><button id=\"trigger\">Edit</button><div id=\"modalContainer\"></div></body></html>",
+        ),
+      );
+      cleanups.push(
+        stubFetch(
+          new Map([["components/edit-video-modal.html", markup]]),
+        ),
+      );
+
+      setFeatureDesignSystemEnabled(designSystemEnabled);
+
+      const modal = new EditModal({
+        removeTrackingScripts: () => {},
+        setGlobalModalState: () => {},
+        sanitizers: {},
+        escapeHtml: (value) => String(value ?? ""),
+        showError: () => {},
+        getMode: () => "live",
+      });
+      await modal.load();
+      applyDesignSystemAttributes(document);
+
+      const trigger = document.getElementById("trigger");
+      assert.ok(trigger);
+
+      await modal.open(
+        {
+          id: "event1",
+          title: "Example",
+          url: "https://example.com/video.mp4",
+          magnet: "",
+          ws: "",
+          xs: "",
+          enableComments: true,
+          isPrivate: false,
+          isNsfw: false,
+          isForKids: false,
+        },
+        { triggerElement: trigger },
+      );
+      await flushMicrotasks();
+
+      const overlay = modal.overlay;
+      assert.ok(overlay, "expected edit modal overlay");
+
+      overlay.dispatchEvent(
+        new window.MouseEvent("click", { bubbles: true, cancelable: true }),
+      );
+      await flushMicrotasks();
+
+      const root = modal.getRoot();
+      assert.ok(root?.classList.contains("hidden"));
+      assert.strictEqual(document.activeElement, trigger);
+
+      t.after(() => {
+        modal.destroy();
+        resetRuntimeFlags();
+        cleanups.reverse().forEach((fn) => fn?.());
+      });
+    },
+  );
+
+  test(
+    `[${modeLabel}] EditModal visibility toggle updates button state`,
+    async (t) => {
+      const markup = await editMarkupPromise;
+      const cleanups = [];
+      cleanups.push(
+        installDom(
+          "<!DOCTYPE html><html><body><div id=\"modalContainer\"></div></body></html>",
+        ),
+      );
+      cleanups.push(
+        stubFetch(
+          new Map([["components/edit-video-modal.html", markup]]),
+        ),
+      );
+
+      setFeatureDesignSystemEnabled(designSystemEnabled);
+
+      const modal = new EditModal({
+        removeTrackingScripts: () => {},
+        setGlobalModalState: () => {},
+        sanitizers: {},
+        escapeHtml: (value) => String(value ?? ""),
+        showError: () => {},
+        getMode: () => "live",
+      });
+      await modal.load();
+      applyDesignSystemAttributes(document);
+
+      await modal.open(
+        {
+          id: "event1",
+          title: "Example",
+          url: "https://example.com/video.mp4",
+          magnet: "",
+          ws: "",
+          xs: "",
+          enableComments: true,
+          isPrivate: false,
+          isNsfw: false,
+          isForKids: false,
+        },
+        {},
+      );
+      await flushMicrotasks();
+
+      const privateButton = modal.visibility.buttons.find((button) =>
+        button?.dataset?.visibilityOption === "private",
+      );
+      const publicButton = modal.visibility.buttons.find((button) =>
+        button?.dataset?.visibilityOption === "public",
+      );
+      assert.ok(privateButton && publicButton);
+
+      const unlockVisibilityButton =
+        modal.root?.querySelector("[data-edit-target=\"editVideoIsPrivate\"]");
+      assert.ok(unlockVisibilityButton);
+
+      unlockVisibilityButton.dispatchEvent(
+        new window.Event("click", { bubbles: true, cancelable: true }),
+      );
+
+      modal.setVisibility("private");
+      assert.equal(privateButton.getAttribute("aria-pressed"), "true");
+      assert.equal(publicButton.getAttribute("aria-pressed"), "false");
+      assert.equal(modal.visibility.container?.dataset.state, "private");
+
+      modal.setVisibility("public");
+      assert.equal(publicButton.getAttribute("aria-pressed"), "true");
+      assert.equal(privateButton.getAttribute("aria-pressed"), "false");
+      assert.equal(modal.visibility.container?.dataset.state, "public");
+
+      t.after(() => {
+        modal.destroy();
+        resetRuntimeFlags();
+        cleanups.reverse().forEach((fn) => fn?.());
+      });
+    },
+  );
+}
 
 test("RevertModal Escape closes and restores trigger focus", async () => {
   const markup = await revertMarkupPromise;
diff --git a/tests/profile-modal-controller.test.mjs b/tests/profile-modal-controller.test.mjs
index f41b8579..3ec2ba73 100644
--- a/tests/profile-modal-controller.test.mjs
+++ b/tests/profile-modal-controller.test.mjs
@@ -3,6 +3,11 @@ import { strict as assert } from 'node:assert';
 import { readFile } from 'node:fs/promises';
 import { JSDOM } from 'jsdom';
 import ProfileModalController from '../js/ui/profileModalController.js';
+import {
+  setFeatureDesignSystemEnabled,
+  resetRuntimeFlags,
+} from '../js/constants.js';
+import { applyDesignSystemAttributes } from '../js/designSystem.js';
 
 const profileModalHtml = await readFile(
   new URL('../components/profile-modal.html', import.meta.url),
@@ -15,6 +20,12 @@ const defaultActorHex = 'a'.repeat(64);
 let dom;
 let container;
 
+async function waitForAnimationFrame(window, cycles = 1) {
+  for (let i = 0; i < cycles; i += 1) {
+    await new Promise((resolve) => window.requestAnimationFrame(() => resolve()));
+  }
+}
+
 beforeEach(() => {
   dom = new JSDOM(
     '<!DOCTYPE html><html><body><div id="modalMount"></div></body></html>',
@@ -163,6 +174,129 @@ function createController(options = {}) {
   });
 }
 
+for (const designSystemEnabled of [false, true]) {
+  const modeLabel = designSystemEnabled ? 'design-system' : 'legacy';
+
+  test(
+    `[${modeLabel}] Profile modal Escape closes and restores trigger focus`,
+    async (t) => {
+      setFeatureDesignSystemEnabled(designSystemEnabled);
+      const controller = createController();
+      await controller.load();
+      applyDesignSystemAttributes(document);
+
+      const trigger = document.createElement('button');
+      trigger.id = 'profileTrigger';
+      trigger.textContent = 'Open profile';
+      document.body.appendChild(trigger);
+      trigger.focus();
+      await waitForAnimationFrame(window, 1);
+
+      const cleanup = () => {
+        try {
+          controller.hide({ silent: true });
+        } catch {}
+        try {
+          trigger.remove();
+        } catch {}
+      };
+
+      let cleanupRan = false;
+      try {
+        await controller.show('account');
+        await waitForAnimationFrame(window, 2);
+
+        const focusTrap =
+          controller.focusTrapContainer ||
+          controller.profileModalPanel ||
+          controller.profileModal ||
+          document.getElementById('profileModal');
+        assert.ok(focusTrap, 'focus trap container should exist');
+
+        focusTrap.dispatchEvent(
+          new window.KeyboardEvent('keydown', {
+            key: 'Escape',
+            bubbles: true,
+            cancelable: true,
+          }),
+        );
+
+        await waitForAnimationFrame(window, 2);
+
+        const modalRoot = document.getElementById('profileModal');
+        assert.ok(modalRoot?.classList.contains('hidden'));
+        assert.strictEqual(document.activeElement, trigger);
+      } finally {
+        cleanup();
+        cleanupRan = true;
+      }
+
+      t.after(() => {
+        if (!cleanupRan) {
+          cleanup();
+        }
+        resetRuntimeFlags();
+      });
+    },
+  );
+
+  test(
+    `[${modeLabel}] Profile modal navigation buttons toggle active state`,
+    async (t) => {
+      setFeatureDesignSystemEnabled(designSystemEnabled);
+      const controller = createController();
+      await controller.load();
+      applyDesignSystemAttributes(document);
+
+      let cleanupRan = false;
+      const cleanup = () => {
+        try {
+          controller.hide({ silent: true });
+        } catch {}
+        cleanupRan = true;
+      };
+
+      try {
+        await controller.show('account');
+        await waitForAnimationFrame(window, 2);
+
+        const accountButton = document.getElementById('profileNavAccount');
+        const relaysButton = document.getElementById('profileNavRelays');
+        const accountPane = document.getElementById('profilePaneAccount');
+        const relaysPane = document.getElementById('profilePaneRelays');
+
+        assert.ok(accountButton && relaysButton);
+        assert.ok(accountPane && relaysPane);
+
+        controller.selectPane('relays');
+        await waitForAnimationFrame(window, 1);
+
+        assert.equal(relaysButton?.getAttribute('aria-selected'), 'true');
+        assert.equal(accountButton?.getAttribute('aria-selected'), 'false');
+        assert.equal(relaysPane?.classList.contains('hidden'), false);
+        assert.equal(accountPane?.classList.contains('hidden'), true);
+
+        controller.selectPane('account');
+        await waitForAnimationFrame(window, 1);
+
+        assert.equal(accountButton?.getAttribute('aria-selected'), 'true');
+        assert.equal(relaysButton?.getAttribute('aria-selected'), 'false');
+        assert.equal(accountPane?.classList.contains('hidden'), false);
+        assert.equal(relaysPane?.classList.contains('hidden'), true);
+      } finally {
+        cleanup();
+      }
+
+      t.after(() => {
+        if (!cleanupRan) {
+          cleanup();
+        }
+        resetRuntimeFlags();
+      });
+    },
+  );
+}
+
 test('load() injects markup and caches expected elements', async () => {
   const controller = createController();
   const result = await controller.load();
diff --git a/tests/video-modal-accessibility.test.mjs b/tests/video-modal-accessibility.test.mjs
index 729eeb1a..f2144d03 100644
--- a/tests/video-modal-accessibility.test.mjs
+++ b/tests/video-modal-accessibility.test.mjs
@@ -4,17 +4,22 @@ import { readFile } from "node:fs/promises";
 import { JSDOM } from "jsdom";
 
 import { VideoModal } from "../js/ui/components/VideoModal.js";
+import {
+  setFeatureDesignSystemEnabled,
+  resetRuntimeFlags,
+} from "../js/constants.js";
+import { applyDesignSystemAttributes } from "../js/designSystem.js";
 
 const modalMarkupPromise = readFile(
   new URL("../components/video-modal.html", import.meta.url),
   "utf8"
 );
 
-async function setupModal() {
+async function setupModal({ designSystemEnabled = false } = {}) {
   const markup = await modalMarkupPromise;
   const dom = new JSDOM(
     `<!DOCTYPE html><html><body><button id="trigger" type="button">Open modal</button><div id="modalContainer">${markup}</div></body></html>`,
-    { url: "https://example.com" }
+    { url: "https://example.com", pretendToBeVisual: true }
   );
 
   const { window } = dom;
@@ -31,6 +36,11 @@ async function setupModal() {
   globalThis.EventTarget = window.EventTarget;
   globalThis.navigator = window.navigator;
   globalThis.location = window.location;
+  globalThis.KeyboardEvent = window.KeyboardEvent;
+  globalThis.MouseEvent = window.MouseEvent;
+
+  setFeatureDesignSystemEnabled(designSystemEnabled);
+  applyDesignSystemAttributes(document);
 
   const modal = new VideoModal({
     removeTrackingScripts: () => {},
@@ -42,15 +52,40 @@ async function setupModal() {
   const playerModal = document.querySelector("#playerModal");
   assert.ok(playerModal, "player modal markup should exist");
   modal.hydrate(playerModal);
+  applyDesignSystemAttributes(playerModal);
 
   const trigger = document.getElementById("trigger");
   assert.ok(trigger, "trigger button should exist");
 
-  return { window, document, modal, playerModal, trigger };
+  const cleanup = () => {
+    try {
+      resetRuntimeFlags();
+    } catch (error) {
+      console.warn("[tests] failed to reset runtime flags", error);
+    }
+    dom.window.close();
+    delete globalThis.window;
+    delete globalThis.document;
+    delete globalThis.HTMLElement;
+    delete globalThis.HTMLVideoElement;
+    delete globalThis.Element;
+    delete globalThis.CustomEvent;
+    delete globalThis.Event;
+    delete globalThis.Node;
+    delete globalThis.EventTarget;
+    delete globalThis.navigator;
+    delete globalThis.location;
+    delete globalThis.KeyboardEvent;
+    delete globalThis.MouseEvent;
+  };
+
+  return { window, document, modal, playerModal, trigger, cleanup };
 }
 
-test("backdrop data-dismiss closes the modal and restores focus", async () => {
-  const { window, document, modal, playerModal, trigger } = await setupModal();
+test("backdrop data-dismiss closes the modal and restores focus", async (t) => {
+  const { window, document, modal, playerModal, trigger, cleanup } =
+    await setupModal();
+  t.after(cleanup);
   const backdrop = playerModal.querySelector("[data-dismiss]");
   assert.ok(backdrop, "modal backdrop should be present");
 
@@ -73,8 +108,10 @@ test("backdrop data-dismiss closes the modal and restores focus", async () => {
   assert.strictEqual(document.activeElement, trigger);
 });
 
-test("Escape key closes the modal and returns focus to the trigger", async () => {
-  const { window, document, modal, playerModal, trigger } = await setupModal();
+test("Escape key closes the modal and returns focus to the trigger", async (t) => {
+  const { window, document, modal, playerModal, trigger, cleanup } =
+    await setupModal();
+  t.after(cleanup);
   const closeButton = playerModal.querySelector("#closeModal");
   assert.ok(closeButton, "close button should be present");
 
@@ -102,3 +139,140 @@ test("Escape key closes the modal and returns focus to the trigger", async () =>
   assert.equal(closeEvents, 1);
   assert.strictEqual(document.activeElement, trigger);
 });
+
+for (const designSystemEnabled of [false, true]) {
+  const modeLabel = designSystemEnabled ? "design-system" : "legacy";
+
+  test(
+    `[${modeLabel}] video modal sticky navigation responds to scroll direction`,
+    async (t) => {
+      const {
+        window,
+        document,
+        modal,
+        playerModal,
+        trigger,
+        cleanup,
+      } = await setupModal({ designSystemEnabled });
+      t.after(cleanup);
+
+      modal.open(null, { triggerElement: trigger });
+      await Promise.resolve();
+
+      const nav = document.getElementById("modalNav");
+      assert.ok(nav, "navigation bar should exist");
+      const scrollRegion = playerModal.querySelector(".bv-modal__panel");
+      assert.ok(scrollRegion, "modal panel should exist");
+
+      let scrollPosition = 0;
+      Object.defineProperty(scrollRegion, "scrollTop", {
+        configurable: true,
+        get() {
+          return scrollPosition;
+        },
+        set(value) {
+          scrollPosition = Number(value) || 0;
+        }
+      });
+
+      scrollRegion.scrollTop = 120;
+      assert.equal(modal.scrollRegion.scrollTop, 120);
+      modal.modalNavScrollHandler?.();
+      assert.equal(nav.style.transform, "translateY(-100%)");
+
+      scrollRegion.scrollTop = 60;
+      assert.equal(modal.scrollRegion.scrollTop, 60);
+      modal.modalNavScrollHandler?.();
+      assert.equal(nav.style.transform, "translateY(0)");
+
+      scrollRegion.scrollTop = 10;
+      assert.equal(modal.scrollRegion.scrollTop, 10);
+      modal.modalNavScrollHandler?.();
+      assert.equal(nav.style.transform, "translateY(0)");
+    }
+  );
+
+  test(
+    `[${modeLabel}] video modal toggles document scroll locking on open/close`,
+    async (t) => {
+      const { document, modal, trigger, cleanup } = await setupModal({
+        designSystemEnabled,
+      });
+      t.after(cleanup);
+
+      modal.open(null, { triggerElement: trigger });
+      await Promise.resolve();
+
+      assert.equal(
+        document.documentElement.classList.contains("modal-open"),
+        true
+      );
+      assert.equal(document.body.classList.contains("modal-open"), true);
+
+      modal.close();
+      await Promise.resolve();
+
+      assert.equal(
+        document.documentElement.classList.contains("modal-open"),
+        false
+      );
+      assert.equal(document.body.classList.contains("modal-open"), false);
+      assert.strictEqual(document.activeElement, trigger);
+    }
+  );
+
+  test(
+    `[${modeLabel}] video modal zap dialog updates aria state while toggling`,
+    async (t) => {
+      const {
+        window,
+        document,
+        modal,
+        playerModal,
+        trigger,
+        cleanup,
+      } = await setupModal({ designSystemEnabled });
+      t.after(cleanup);
+
+      modal.open(null, { triggerElement: trigger });
+      await Promise.resolve();
+
+      const zapButton = document.getElementById("modalZapBtn");
+      const zapDialog = document.getElementById("modalZapDialog");
+      const amountInput = document.getElementById("modalZapAmountInput");
+      assert.ok(zapButton, "zap trigger should exist");
+      assert.ok(zapDialog, "zap dialog should exist");
+      assert.ok(amountInput, "zap amount input should exist");
+
+      modal.setZapVisibility(true);
+
+      assert.equal(zapDialog.hidden, true);
+      assert.equal(zapDialog.dataset.state, "closed");
+      assert.equal(zapButton.getAttribute("aria-expanded"), "false");
+
+      zapButton.dispatchEvent(
+        new window.MouseEvent("click", { bubbles: true, cancelable: true })
+      );
+
+      assert.equal(zapDialog.hidden, false);
+      assert.equal(zapDialog.dataset.state, "open");
+      assert.equal(zapDialog.getAttribute("aria-hidden"), "false");
+      assert.equal(zapButton.getAttribute("aria-expanded"), "true");
+      assert.strictEqual(document.activeElement, amountInput);
+
+      const closeButton = document.getElementById("modalZapCloseBtn");
+      assert.ok(closeButton, "zap close button should exist");
+
+      closeButton.dispatchEvent(
+        new window.MouseEvent("click", { bubbles: true, cancelable: true })
+      );
+
+      assert.equal(zapDialog.hidden, true);
+      assert.equal(zapDialog.dataset.state, "closed");
+      assert.equal(zapDialog.getAttribute("aria-hidden"), "true");
+      assert.equal(zapButton.getAttribute("aria-expanded"), "false");
+
+      modal.close();
+    }
+  );
+}