diff --git a/tests/test_snapshot_units.py b/tests/test_snapshot_units.py
index 4c9472c..12492d8 100644
--- a/tests/test_snapshot_units.py
+++ b/tests/test_snapshot_units.py
@@ -2,9 +2,10 @@
 """Focused unit tests for snapshot.py helper functions and their error/
 fallback paths. These run as plain unittests (no Percy CLI needed) and
 target branches the end-to-end suite in test_snapshot.py does not reach."""
+import importlib
 import os
 import unittest
-from unittest.mock import patch, MagicMock
+from unittest.mock import patch, MagicMock, Mock
 
 import percy.snapshot as local
 
@@ -115,5 +116,331 @@ def test_invalid_min_height_falls_back_to_window_height(self):
         self.assertEqual(result, [])
 
 
+class TestResizeSettleSecondsParsing(unittest.TestCase):
+    def test_invalid_env_value_falls_back_to_default(self):
+        # Module-level parse: an unparseable PERCY_RESIZE_SETTLE_SECONDS must
+        # fall back to the 0.5s default instead of raising at import time.
+        try:
+            with patch.dict(os.environ, {'PERCY_RESIZE_SETTLE_SECONDS': 'not-a-float'}):
+                importlib.reload(local)
+                self.assertEqual(local.RESIZE_SETTLE_SECONDS, 0.5)
+        finally:
+            # Restore module globals from the real (unpatched) environment.
+            importlib.reload(local)
+
+
+class TestShouldSkipIframeEdgeCases(unittest.TestCase):
+    @staticmethod
+    def _meta(src, percy_id):
+        return {
+            'src': src, 'srcdoc': None, 'percyElementId': percy_id,
+            'dataPercyIgnore': False, 'matchesIgnoreSelector': False, 'index': 0,
+        }
+
+    def test_unparseable_origin_is_skipped(self):
+        # src passes the unsupported-scheme check but has no scheme://netloc,
+        # so get_origin returns None -> "invalid URL" skip branch.
+        iframe = self._meta('not-a-url', 'pid-1')
+        self.assertTrue(local._should_skip_iframe(iframe, 'http://main.example.com'))
+
+    def test_cross_origin_without_percy_element_id_is_skipped(self):
+        iframe = self._meta('https://other.example.com/widget', None)
+        self.assertTrue(local._should_skip_iframe(iframe, 'http://main.example.com'))
+
+
+def _tree_ctx(max_depth=3):
+    return {
+        'max_frame_depth': max_depth,
+        'ignore_selectors': [],
+        'serialize_options': {},
+        'percy_dom_script': 'PERCY_DOM',
+    }
+
+
+class TestProcessFrameTreeGuards(unittest.TestCase):
+    @staticmethod
+    def _meta(src, percy_id, ignore=False):
+        return {
+            'src': src, 'srcdoc': None, 'percyElementId': percy_id,
+            'dataPercyIgnore': ignore, 'matchesIgnoreSelector': False, 'index': 0,
+        }
+
+    def test_depth_beyond_max_returns_empty(self):
+        driver = MagicMock()
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/', 'pid-x'), 4, set(),
+            _tree_ctx(max_depth=3))
+        self.assertEqual(result, [])
+        driver.switch_to.frame.assert_not_called()
+
+    def test_missing_iframe_element_returns_empty(self):
+        driver = MagicMock()
+        # querySelector by data-percy-element-id finds nothing (DOM mutated).
+        driver.execute_script.side_effect = [None]
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/', 'pid-x'), 1, set(),
+            _tree_ctx())
+        self.assertEqual(result, [])
+        driver.switch_to.frame.assert_not_called()
+
+    def test_document_url_read_failure_is_treated_as_unsupported(self):
+        driver = MagicMock()
+        driver.execute_script.side_effect = [
+            Mock(name='iframe_element'),   # querySelector
+            Exception('document.URL blew up'),  # post-switch URL read
+        ]
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/', 'pid-x'), 1, set(),
+            _tree_ctx())
+        self.assertEqual(result, [])
+        driver.switch_to.parent_frame.assert_called_once()
+
+    def test_empty_serialize_result_returns_empty(self):
+        driver = MagicMock()
+        driver.execute_script.side_effect = [
+            Mock(name='iframe_element'),      # querySelector
+            'https://x.example.com/page',     # post-switch document.URL
+            None,                             # inject PercyDOM
+            None,                             # serialize returned nothing
+        ]
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/page', 'pid-x'), 1, set(),
+            _tree_ctx())
+        self.assertEqual(result, [])
+
+    def test_nested_enumeration_failure_keeps_parent_capture(self):
+        driver = MagicMock()
+        driver.execute_script.side_effect = [
+            Mock(name='iframe_element'),
+            'https://x.example.com/page',
+            None,
+            {'snapshot': {'html': '<x/>'}, 'frameUrl': 'https://x.example.com/page'},
+            Exception('nested enumerate blew up'),
+        ]
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/page', 'pid-x'), 1, set(),
+            _tree_ctx())
+        self.assertEqual(len(result), 1)
+        self.assertEqual(result[0]['iframeData']['percyElementId'], 'pid-x')
+
+    def test_skipped_nested_child_is_not_recursed_into(self):
+        driver = MagicMock()
+        driver.execute_script.side_effect = [
+            Mock(name='iframe_element'),
+            'https://x.example.com/page',
+            None,
+            {'snapshot': {'html': '<x/>'}, 'frameUrl': 'https://x.example.com/page'},
+            # nested enumeration returns one child carrying data-percy-ignore
+            [self._meta('https://c.example.com/', 'pid-c', ignore=True)],
+        ]
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/page', 'pid-x'), 1, set(),
+            _tree_ctx())
+        self.assertEqual(len(result), 1)
+        # Only one frame switch happened — the ignored child was never entered.
+        self.assertEqual(driver.switch_to.frame.call_count, 1)
+
+    def test_nested_child_capture_is_collected(self):
+        driver = MagicMock()
+        driver.execute_script.side_effect = [
+            # depth-1 frame
+            Mock(name='iframe_element_x'),
+            'https://x.example.com/page',
+            None,
+            {'snapshot': {'html': '<x/>'}, 'frameUrl': 'https://x.example.com/page'},
+            [self._meta('https://c.example.com/', 'pid-c')],
+            # nested depth-2 frame
+            Mock(name='iframe_element_c'),
+            'https://c.example.com/',
+            None,
+            {'snapshot': {'html': '<c/>'}, 'frameUrl': 'https://c.example.com/'},
+            [],
+        ]
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/page', 'pid-x'), 1, set(),
+            _tree_ctx())
+        pids = [e['iframeData']['percyElementId'] for e in result]
+        self.assertEqual(pids, ['pid-x', 'pid-c'])
+
+    def test_default_content_failure_is_swallowed_at_depth_1(self):
+        driver = MagicMock()
+        driver.execute_script.side_effect = [
+            Mock(name='iframe_element'),
+            'https://x.example.com/page',
+            Exception('inject blew up'),
+        ]
+        driver.switch_to.parent_frame.side_effect = Exception('lost parent')
+        driver.switch_to.default_content.side_effect = Exception('lost everything')
+        # At depth 1 a total context loss must not raise — the walk just ends.
+        result = local.process_frame_tree(
+            driver, self._meta('https://x.example.com/page', 'pid-x'), 1, set(),
+            _tree_ctx())
+        self.assertEqual(result, [])
+
+    def test_context_lost_links_original_error_as_cause(self):
+        driver = MagicMock()
+        driver.execute_script.side_effect = [
+            Mock(name='iframe_element'),
+            'https://c.example.com/',
+            Exception('inject blew up'),
+        ]
+        driver.switch_to.parent_frame.side_effect = Exception('lost parent')
+        # depth > 1 with a captured processing error: PercyContextLost is
+        # raised and chains the original error for diagnosis.
+        with self.assertRaises(local.PercyContextLost) as cm:
+            local.process_frame_tree(
+                driver, self._meta('https://c.example.com/', 'pid-c'), 2, set(),
+                _tree_ctx())
+        self.assertEqual(cm.exception.partial_capture, [])
+        self.assertIsNotNone(cm.exception.__cause__)
+
+
+class TestCaptureCorsIframesErrorHandling(unittest.TestCase):
+    def test_unexpected_error_returns_empty_list(self):
+        driver = MagicMock()
+        # Enumeration "succeeds" but yields a malformed (non-dict) entry;
+        # the resulting AttributeError is swallowed by the outer guard.
+        driver.execute_script.side_effect = [[None]]
+        result = local._capture_cors_iframes(
+            driver, 'http://main.example.com/', _tree_ctx())
+        self.assertEqual(result, [])
+
+
+class TestExposeClosedShadowRootsEdgeCases(unittest.TestCase):
+    def test_capabilities_access_failure_is_treated_as_non_chromium(self):
+        cdp_calls = []
+
+        class StubDriver:
+            @property
+            def capabilities(self):
+                raise Exception('capabilities unavailable')
+
+            def execute_cdp_cmd(self, cmd, params):
+                cdp_calls.append((cmd, params))
+
+        local.expose_closed_shadow_roots(StubDriver())
+        self.assertEqual(cdp_calls, [])
+
+    def test_missing_object_ids_are_skipped(self):
+        driver = MagicMock()
+        driver.capabilities = {'browserName': 'chrome'}
+
+        def cdp(cmd, _params):
+            if cmd == 'DOM.getDocument':
+                return {'root': {
+                    'backendNodeId': 1,
+                    'shadowRoots': [{'backendNodeId': 2, 'shadowRootType': 'closed'}],
+                    'children': [],
+                }}
+            if cmd == 'DOM.resolveNode':
+                return {'object': {}}  # no objectId -> pair is skipped
+            return {}
+        driver.execute_cdp_cmd.side_effect = cdp
+
+        local.expose_closed_shadow_roots(driver)
+        cdp_cmds = [c.args[0] for c in driver.execute_cdp_cmd.call_args_list]
+        self.assertNotIn('Runtime.callFunctionOn', cdp_cmds)
+
+    def test_resolve_node_failure_is_swallowed(self):
+        driver = MagicMock()
+        driver.capabilities = {'browserName': 'chrome'}
+
+        def cdp(cmd, _params):
+            if cmd == 'DOM.getDocument':
+                return {'root': {
+                    'backendNodeId': 1,
+                    'shadowRoots': [{'backendNodeId': 2, 'shadowRootType': 'closed'}],
+                    'children': [],
+                }}
+            if cmd == 'DOM.resolveNode':
+                raise Exception('node gone')
+            return {}
+        driver.execute_cdp_cmd.side_effect = cdp
+
+        local.expose_closed_shadow_roots(driver)  # must not raise
+
+    def test_get_document_failure_and_dom_disable_failure_are_swallowed(self):
+        driver = MagicMock()
+        driver.capabilities = {'browserName': 'chrome'}
+
+        def cdp(cmd, _params):
+            if cmd == 'DOM.enable':
+                return {}
+            raise Exception(f'{cmd} failed')  # DOM.getDocument AND DOM.disable
+        driver.execute_cdp_cmd.side_effect = cdp
+
+        local.expose_closed_shadow_roots(driver)  # must not raise
+
+
+class TestGetOriginHelpers(unittest.TestCase):
+    def test_get_origin_returns_none_without_scheme_or_netloc(self):
+        self.assertIsNone(local.get_origin('not-a-url'))
+        self.assertIsNone(local.get_origin('http://'))
+
+    def test_get_origin_returns_none_when_parsing_raises(self):
+        # urlparse raises AttributeError on non-str input
+        self.assertIsNone(local.get_origin(123))
+
+    def test_get_origin_compat_shim_returns_empty_string(self):
+        self.assertEqual(local._get_origin('https://a.example.com/x'),
+                         'https://a.example.com')
+        self.assertEqual(local._get_origin('not-a-url'), '')
+
+
+class TestGetSerializedDomPageUrlFailure(unittest.TestCase):
+    def test_current_url_failure_degrades_to_no_page_url(self):
+        class StubDriver:
+            def __init__(self):
+                self.calls = 0
+
+            def execute_script(self, _script, *_args):
+                self.calls += 1
+                if self.calls == 1:
+                    return {'html': '<html/>'}  # main serialize
+                return []  # iframe enumeration
+
+            @property
+            def current_url(self):
+                raise Exception('no url available')
+
+        dom = local.get_serialized_dom(
+            StubDriver(), [{'name': 'k', 'value': 'v'}],
+            percy_dom_script='PERCY_DOM')
+        self.assertNotIn('corsIframes', dom)
+        self.assertEqual(dom['cookies'], [{'name': 'k', 'value': 'v'}])
+
+
+class TestResponsiveDebugDump(unittest.TestCase):
+    def tearDown(self):
+        if os.path.exists('output_file.json'):
+            os.remove('output_file.json')
+
+    @staticmethod
+    def _driver():
+        driver = MagicMock()
+        driver.get_window_size.return_value = {'width': 1000, 'height': 800}
+        return driver
+
+    @patch('percy.snapshot.PERCY_DEBUG', True)
+    @patch('percy.snapshot.change_window_dimension_and_wait', MagicMock())
+    @patch('percy.snapshot._wait_for_ready', MagicMock(return_value=None))
+    @patch('percy.snapshot.get_responsive_widths', MagicMock(return_value=[]))
+    def test_debug_dump_is_written_when_percy_debug(self):
+        result = local.capture_responsive_dom(
+            self._driver(), {}, {}, percy_dom_script='PERCY_DOM')
+        self.assertEqual(result, [])
+        self.assertTrue(os.path.exists('output_file.json'))
+
+    @patch('percy.snapshot.PERCY_DEBUG', True)
+    @patch('percy.snapshot.change_window_dimension_and_wait', MagicMock())
+    @patch('percy.snapshot._wait_for_ready', MagicMock(return_value=None))
+    @patch('percy.snapshot.get_responsive_widths', MagicMock(return_value=[]))
+    @patch('percy.snapshot.json.dump', side_effect=TypeError('not serializable'))
+    def test_debug_dump_failure_is_logged_not_raised(self, _mock_dump):
+        result = local.capture_responsive_dom(
+            self._driver(), {}, {}, percy_dom_script='PERCY_DOM')
+        self.assertEqual(result, [])
+
+
 if __name__ == '__main__':
     unittest.main()