preview: improve sidebar layout and screenshot export

.github/scripts/test_package_release.py

@@ -30,6 +30,9 @@ class PackagingTests(unittest.TestCase):
         "docs/README.zh-CN.md",
         "docs/CONTRIBUTING.zh-CN.md",
         "preview/index.html",
+        "preview/lib/html2canvas/html2canvas.min.js",
+        "preview/lib/html2canvas/LICENSE",
+        "preview/lib/html2canvas/README.md",
         "docs/images/README.md",
     )
 
@@ -130,8 +133,15 @@ class PackagingTests(unittest.TestCase):
         self.assertEqual(expected_members, zipped)
         self.assertEqual(expected_members, tarred)
         self.assertIn(prefix + "themes/demo/mail/base/head.tmpl", zipped)
+        for name in ("html2canvas.min.js", "LICENSE", "README.md"):
+            self.assertIn(prefix + "preview/lib/html2canvas/" + name, zipped)
         self.assertFalse(any("stale" in name or "/build/" in name for name in zipped))
 
+    def test_missing_screenshot_library_is_rejected(self):
+        (self.root / "preview/lib/html2canvas/html2canvas.min.js").unlink()
+        with self.assertRaisesRegex(ValueError, "Expected regular file"):
+            PACKER.collect(self.root, self.VERSION)
+
     def test_existing_archives_are_not_overwritten(self):
         PACKER.package(self.root, self.VERSION, self.output)
         original_archives = {