diff --git a/static/tests/frontend-new/specs/pageupdown.spec.ts b/static/tests/frontend-new/specs/pageupdown.spec.ts
new file mode 100644
index 0000000..12034c0
--- /dev/null
+++ b/static/tests/frontend-new/specs/pageupdown.spec.ts
@@ -0,0 +1,168 @@
+import {expect, test} from '@playwright/test';
+import {clearPadContent, getPadBody, goToNewPad, writeToPad}
+    from 'ep_etherpad-lite/tests/frontend-new/helper/padHelper';
+
+test.beforeEach(async ({page}) => {
+  await goToNewPad(page);
+});
+
+// Mirror the helper used in font_size.spec: set the underlying <select>
+// value and fire a bubbling change event so niceSelect passes it through.
+const setFontSizeIndex = (page: any, index: string) =>
+  page.evaluate((i: string) => {
+    const sel = document.querySelector<HTMLSelectElement>('#font-size')!;
+    sel.value = i;
+    sel.dispatchEvent(new Event('change', {bubbles: true}));
+  }, index);
+
+// Return the 0-based index of the div that currently contains the caret.
+const getCaretLineIndex = (page: any): Promise<number> =>
+  page.frame('ace_inner')!.evaluate(() => {
+    const sel = document.getSelection();
+    if (!sel || !sel.focusNode) return -1;
+    let node = sel.focusNode as HTMLElement;
+    while (node && node.tagName !== 'DIV') node = node.parentElement!;
+    if (!node) return -1;
+    return Array.from(document.getElementById('innerdocbody')!.children).indexOf(node);
+  });
+
+test.describe('ep_font_size – Page Up / Page Down with mixed font sizes', () => {
+  // Allow up to 2 retries to account for timing variance in headless browsers.
+  test.describe.configure({retries: 2});
+
+  /**
+   * Build a pad with 60 lines and then inject random-looking font-size spans
+   * into every third div, directly inside ace_inner.  This reproduces the
+   * markup that ep_font_size's aceCreateDomLine / aceAttribsToClasses hooks
+   * emit at runtime (e.g. `<span class="font-size:14" style="font-size:14px">`)
+   * without having to select each line individually in the UI, which would
+   * make the setup prohibitively slow.
+   */
+  const buildPadWithMixedFontSizes = async (page: any) => {
+    await clearPadContent(page);
+    for (let i = 0; i < 60; i++) {
+      await writeToPad(page, `line ${i + 1}`);
+      await page.keyboard.press('Enter');
+    }
+
+    const innerFrame = page.frame('ace_inner')!;
+    await innerFrame.evaluate(() => {
+      // A subset of the sizes exposed by the plugin (from shared.js).
+      const sizes = [8, 10, 12, 14, 18, 24, 30, 40, 60];
+      const body = document.getElementById('innerdocbody')!;
+      Array.from(body.children).forEach((div, i) => {
+        if (i % 3 !== 0) return; // leave two-thirds of lines at default size
+        const size = sizes[i % sizes.length];
+        (div as HTMLElement).style.fontSize = `${size}px`;
+        // Wrap the text node in a span that carries the same class attribute
+        // and inline style that ep_font_size produces.
+        const span = document.createElement('span');
+        span.className = `font-size:${size}`;
+        span.style.fontSize = `${size}px`;
+        while (div.firstChild) span.appendChild(div.firstChild);
+        div.appendChild(span);
+      });
+    });
+    await page.waitForTimeout(500);
+  };
+
+  // ── Tests adapted from the Etherpad-core pageupdown.js spec (PR #4622) ──
+
+  test('PageDown with mixed font sizes moves caret forward', async ({page}) => {
+    await buildPadWithMixedFontSizes(page);
+
+    // Move caret to the very first line.
+    await page.keyboard.down('Control');
+    await page.keyboard.press('Home');
+    await page.keyboard.up('Control');
+    await page.waitForTimeout(200);
+
+    await page.keyboard.press('PageDown');
+    await page.waitForTimeout(1000);
+
+    const line = await getCaretLineIndex(page);
+    expect(line).toBeGreaterThan(2);
+  });
+
+  test('PageUp with mixed font sizes moves caret backward', async ({page}) => {
+    await buildPadWithMixedFontSizes(page);
+
+    // Move caret to the very last line.
+    await page.keyboard.down('Control');
+    await page.keyboard.press('End');
+    await page.keyboard.up('Control');
+    await page.waitForTimeout(200);
+
+    await page.keyboard.press('PageUp');
+    await page.waitForTimeout(500);
+
+    const line = await getCaretLineIndex(page);
+    expect(line).toBeLessThan(55);
+  });
+
+  test('PageDown then PageUp with mixed font sizes returns near start', async ({page}) => {
+    await buildPadWithMixedFontSizes(page);
+
+    // Start at the top.
+    await page.keyboard.down('Control');
+    await page.keyboard.press('Home');
+    await page.keyboard.up('Control');
+    await page.waitForTimeout(200);
+
+    await page.keyboard.press('PageDown');
+    await page.waitForTimeout(1000);
+    await page.keyboard.press('PageUp');
+    await page.waitForTimeout(1000);
+
+    const line = await getCaretLineIndex(page);
+    // Allow some drift due to viewport calculations with varied font sizes.
+    expect(line).toBeLessThan(8);
+  });
+
+  // ── Plugin-UI font size + PageDown scrolls the viewport ──
+
+  test('plugin font size applied via UI keeps PageDown navigation working', async ({page}) => {
+    const padBody = await getPadBody(page);
+    await clearPadContent(page);
+    for (let i = 0; i < 60; i++) {
+      await writeToPad(page, `line ${i + 1}`);
+      await page.keyboard.press('Enter');
+    }
+
+    // Select all text and apply a large font size through the plugin UI so
+    // PageDown must move the viewport, not only the caret.
+    await page.keyboard.down('Control');
+    await page.keyboard.press('A');
+    await page.keyboard.up('Control');
+    await setFontSizeIndex(page, '22'); // index 22 → size 60
+    await page.waitForTimeout(500);
+
+    // Confirm the class was applied to the first span.
+    await expect(padBody.locator('div').first().locator('span').first())
+        .toHaveClass(/font-size:60/);
+
+    // Move caret to the very first line.
+    await page.keyboard.down('Control');
+    await page.keyboard.press('Home');
+    await page.keyboard.up('Control');
+    await page.waitForTimeout(200);
+
+    const lineBefore = await getCaretLineIndex(page);
+    const outerFrame = page.frame('ace_outer')!;
+    const scrollBefore = await outerFrame.evaluate(
+        () => document.querySelector<HTMLElement>('#outerdocbody')!
+            .parentElement!.scrollTop,
+    );
+
+    await page.keyboard.press('PageDown');
+    await page.waitForTimeout(1000);
+
+    const lineAfter = await getCaretLineIndex(page);
+    const scrollAfter = await outerFrame.evaluate(
+        () => document.querySelector<HTMLElement>('#outerdocbody')!
+            .parentElement!.scrollTop,
+    );
+    expect(lineAfter).toBeGreaterThan(lineBefore);
+    expect(scrollAfter).toBeGreaterThanOrEqual(scrollBefore);
+  });
+});