fix(reports): narrow spinner checks to viewport and tighten exception handling

- Replace global '.loading' count check in webdriver.py with a
  getBoundingClientRect viewport-visibility check to avoid deadlock
  when DashboardVirtualization renders off-screen placeholder spinners
- Narrow except clause in screenshot_utils.py from bare Exception to
  PlaywrightTimeout so non-timeout errors (e.g. browser crash) propagate
- Fix load_wait default from 30 s to 60 s to match SCREENSHOT_LOAD_WAIT
  config default
- Add tests covering per-tile spinner wait, timeout warning, non-timeout
  propagation, and load_wait default value

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit_tests/utils/test_screenshot_utils.py

@@ -320,3 +320,61 @@ class TestTakeTiledScreenshot:
             # Each wait should use the scroll settle timeout constant
             for call in mock_page.wait_for_timeout.call_args_list:
                 assert call[0][0] == SCROLL_SETTLE_TIMEOUT_MS
+
+    def test_per_tile_spinner_wait_uses_viewport_check(self, mock_page):
+        """wait_for_function polls viewport-visible spinners after each scroll."""
+        with patch("superset.utils.screenshot_utils.combine_screenshot_tiles"):
+            take_tiled_screenshot(
+                mock_page, "dashboard", tile_height=2000, load_wait=30
+            )
+
+        # 3 tiles → 3 wait_for_function calls, one per tile
+        assert mock_page.wait_for_function.call_count == 3
+
+        # Each call uses viewport-scoped JS and the load_wait timeout
+        for call in mock_page.wait_for_function.call_args_list:
+            js = call[0][0]
+            assert "getBoundingClientRect" in js
+            assert "window.innerHeight" in js
+            assert call[1]["timeout"] == 30 * 1000
+
+    def test_per_tile_spinner_timeout_logs_warning_and_continues(self, mock_page):
+        """A per-tile spinner timeout logs a warning but still takes the screenshot."""
+        from superset.utils.screenshot_utils import PlaywrightTimeout
+
+        timeout = PlaywrightTimeout()
+        mock_page.wait_for_function.side_effect = timeout
+
+        with patch("superset.utils.screenshot_utils.logger") as mock_logger:
+            with patch("superset.utils.screenshot_utils.combine_screenshot_tiles"):
+                result = take_tiled_screenshot(
+                    mock_page, "dashboard", tile_height=2000, load_wait=30
+                )
+
+        # Screenshot should still proceed (non-fatal)
+        assert result is not None
+        # Warning logged for each tile that timed out
+        assert mock_logger.warning.call_count == 3
+        mock_logger.warning.assert_any_call(
+            "Timed out waiting for visible spinners to clear on tile %s/%s",
+            1,
+            3,
+        )
+
+    def test_per_tile_non_timeout_exceptions_propagate(self, mock_page):
+        """Non-timeout exceptions from wait_for_function are not swallowed."""
+        mock_page.wait_for_function.side_effect = RuntimeError("browser crashed")
+
+        with pytest.raises(RuntimeError, match="browser crashed"):
+            take_tiled_screenshot(
+                mock_page, "dashboard", tile_height=2000, load_wait=30
+            )
+
+    def test_load_wait_default_is_sixty_seconds(self):
+        """load_wait defaults to 60 to match SCREENSHOT_LOAD_WAIT config default."""
+        import inspect
+
+        from superset.utils.screenshot_utils import take_tiled_screenshot
+
+        sig = inspect.signature(take_tiled_screenshot)
+        assert sig.parameters["load_wait"].default == 60