diff --git a/.gitignore b/.gitignore
index 47e94a8..402d0ce 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,6 +3,8 @@
 
 # Test / coverage
 /coverage/
+/test-results/
+/playwright-report/
 
 # Dependencies (reuse a sibling install or `npm ci`)
 # No trailing slash so a `node_modules` symlink is ignored too.
diff --git a/README.md b/README.md
index 8201762..b25d937 100644
--- a/README.md
+++ b/README.md
@@ -169,6 +169,22 @@ and the bootstrap — is held at **100/100/100/100** (statements / branches /
 functions / lines). The fetch, crypto, and storage seams are injected, so the
 suite needs no mocking libraries.
 
+### End-to-end (real browser)
+
+happy-dom has no real layout or scrollbars, so render-layer bugs (e.g. the
+editor highlight drifting behind the selection when a scrollbar shrinks the
+textarea's client box) can't be caught by the unit suite. A small Playwright
+harness mounts the real `src/` modules in Chromium for those cases.
+
+```bash
+npx playwright install chromium   # once per machine
+npm run test:e2e
+```
+
+The harness (`tests/e2e/`) serves the repo over HTTP and imports the actual
+source as native ESM — no bundling, always current. It is **not** part of
+`npm test` or the coverage gate.
+
 ## License
 
 Apache-2.0.
diff --git a/package.json b/package.json
index 9e8a25b..bed332f 100644
--- a/package.json
+++ b/package.json
@@ -9,9 +9,11 @@
     "build": "node build/build.mjs",
     "test": "vitest run --coverage --config tests/vitest.config.ts",
     "test:watch": "vitest --config tests/vitest.config.ts",
+    "test:e2e": "playwright test",
     "dev": "node build/build.mjs && python3 -m http.server -d dist 8900"
   },
   "devDependencies": {
+    "@playwright/test": "^1.48.0",
     "@vitest/coverage-v8": "^2.1.8",
     "esbuild": "^0.21.5",
     "happy-dom": "^15.11.0",
diff --git a/playwright.config.js b/playwright.config.js
new file mode 100644
index 0000000..1475f66
--- /dev/null
+++ b/playwright.config.js
@@ -0,0 +1,25 @@
+import { defineConfig } from '@playwright/test';
+
+// Real-browser regression tests. happy-dom (the unit layer) has no scrollbar or
+// real box layout, so render-layer bugs — e.g. the editor's highlight drifting
+// behind the selection when a scrollbar shrinks the textarea's client box —
+// can only be caught in a real engine. These run separately from `npm test`.
+//
+// Setup once per machine: `npx playwright install chromium`.
+// Run: `npm run test:e2e`.
+export default defineConfig({
+  testDir: './tests/e2e',
+  testMatch: '**/*.spec.js',
+  // Serve the repo root over HTTP so the harness can import the *actual* source
+  // modules (/src/**) as native ESM — no bundling, always current.
+  webServer: {
+    command: 'python3 -m http.server -d . 5599',
+    url: 'http://127.0.0.1:5599/tests/e2e/editor.html',
+    reuseExistingServer: !process.env.CI,
+    timeout: 30_000,
+  },
+  use: {
+    baseURL: 'http://127.0.0.1:5599',
+    browserName: 'chromium',
+  },
+});
diff --git a/tests/e2e/editor-alignment.spec.js b/tests/e2e/editor-alignment.spec.js
new file mode 100644
index 0000000..cdcd4c3
--- /dev/null
+++ b/tests/e2e/editor-alignment.spec.js
@@ -0,0 +1,44 @@
+import { test, expect } from '@playwright/test';
+
+// Regression guard for the editor's highlight/selection alignment.
+//
+// The bug: a long line gives the <textarea> a horizontal scrollbar, which
+// shrinks its clientHeight (~10px). That made the textarea's max vertical
+// scrollTop larger than the highlight <pre>'s (overflow:hidden, no scrollbar),
+// so scrolling to the bottom clamped pre.scrollTop below ta.scrollTop and the
+// painted glyphs lagged the native selection — worst on the last line.
+// Repro shape: shift-click a CREATE TABLE, scroll down, select the last line.
+//
+// This can only be caught in a real engine — happy-dom has no scrollbar layout.
+test.describe('editor highlight/selection alignment', () => {
+  test('highlight tracks the textarea scrolled to the bottom with a long line', async ({ page }) => {
+    await page.goto('/tests/e2e/editor.html');
+    await page.waitForFunction(() => window.__ready === true);
+
+    // Tall content whose LAST line is long enough to force a horizontal scrollbar.
+    const sql = [
+      ...Array.from({ length: 30 }, (_, i) => `SELECT col_${i} FROM t WHERE x = ${i}`),
+      `COMMENT '${'x'.repeat(400)}'`,
+    ].join('\n');
+    await page.evaluate((s) => window.__setSql(s), sql);
+
+    const m = await page.evaluate(() => {
+      const ta = window.__app.dom.editorTextarea;
+      const pre = window.__app.dom.editorPre;
+      ta.focus();
+      ta.scrollTop = ta.scrollHeight; // scroll to the very bottom
+      ta.dispatchEvent(new Event('scroll'));
+      return {
+        taClientH: ta.clientHeight,
+        preClientH: pre.clientHeight,
+        taScrollTop: ta.scrollTop,
+        preScrollTop: pre.scrollTop,
+      };
+    });
+
+    // The textarea must not reserve scrollbar space the highlight lacks…
+    expect(m.taClientH).toBe(m.preClientH);
+    // …so the highlight reaches the same offset instead of clamping behind it.
+    expect(Math.abs(m.taScrollTop - m.preScrollTop)).toBeLessThan(1);
+  });
+});
diff --git a/tests/e2e/editor.html b/tests/e2e/editor.html
new file mode 100644
index 0000000..d0a129a
--- /dev/null
+++ b/tests/e2e/editor.html
@@ -0,0 +1,36 @@
+<!doctype html>
+<html>
+<head>
+  <meta charset="utf-8" />
+  <title>editor harness</title>
+  <!-- the real stylesheet, so .sql-textarea / .sql-pre lay out exactly as shipped -->
+  <link rel="stylesheet" href="/src/styles.css" />
+  <style>
+    html, body { margin: 0; background: #111; }
+    /* .sql-editor is height/width:100%, so the mount host needs a fixed box. */
+    #host { width: 680px; height: 340px; }
+  </style>
+</head>
+<body data-theme="dark">
+  <div id="host"></div>
+  <!-- Mount the real editor module with a minimal stub controller. No bundling:
+       the browser resolves the /src ESM graph directly. -->
+  <script type="module">
+    import { mountEditor } from '/src/ui/editor.js';
+    import { createState } from '/src/state.js';
+
+    const state = createState({ loadStr: (k, d) => d, loadJSON: (k, d) => d });
+    const app = { state, dom: {}, actions: { rerenderTabs() {}, updateSaveBtn() {} } };
+    window.__app = app;
+    mountEditor(app, document.getElementById('host'));
+
+    // Set editor content the way typing/replace does (drives paint()).
+    window.__setSql = (sql) => {
+      const ta = app.dom.editorTextarea;
+      ta.value = sql;
+      ta.dispatchEvent(new Event('input', { bubbles: true }));
+    };
+    window.__ready = true;
+  </script>
+</body>
+</html>