test: deterministically tear down example widgets

The namespace from runpy.run_path is discarded, so widgets created by
an example are garbage-collected at an arbitrary later point during
event processing.  A still-visible widget whose C++ side is deleted
mid-event-loop can receive a paintEvent during deletion, which PyQt5
escalates to a process abort:

    RuntimeError: wrapped C/C++ object of type QRangeSlider has been
    deleted

as seen intermittently on the windows-latest (3.12) pyqt5 job in
test_example[demo_widgets/range_slider.py] (e.g. the scheduled main
run on 2026-08-22, and both pyapp-kit#741 and pyapp-kit#742).

Close (hide) all top-level widgets first so nothing can paint, then
deleteLater() + sendPostedEvents(None, DeferredDelete) to destroy the
native widgets (and stop their timers) while hidden -- note that
processEvents() alone does NOT dispatch DeferredDelete events -- then
gc.collect() for anything left over.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: 2 people

tests/test_examples.py

@@ -1,8 +1,10 @@
+import gc
 import runpy
 from pathlib import Path
 from unittest.mock import patch
 
 import pytest
+from qtpy.QtCore import QCoreApplication, QEvent
 from qtpy.QtWidgets import QApplication
 
 EXAMPLES_DIR = Path(__file__).parent.parent / "docs" / "examples"
@@ -34,3 +36,24 @@ def test_example(qapp: QApplication, example: Path) -> None:
                 # dependency it requires, that's fine
                 pytest.xfail(str(e))
             raise
+        finally:
+            # Deterministically tear down whatever the example created.  The
+            # namespace returned by runpy is discarded, so the example's
+            # widgets are otherwise garbage-collected at an arbitrary later
+            # point -- and a still-visible widget whose C++ side is deleted
+            # during event processing can receive a paintEvent mid-deletion,
+            # which aborts the process on Windows/PyQt5 with
+            # "RuntimeError: wrapped C/C++ object ... has been deleted"
+            # (seen intermittently in superqt's QRangeSlider paintEvent for
+            # demo_widgets/range_slider.py).  Hide everything first so
+            # nothing paints, then destroy the native widgets while they are
+            # hidden (which also stops their timers), then collect whatever
+            # is left while flushing events.
+            for widget in qapp.topLevelWidgets():
+                widget.close()
+                widget.deleteLater()
+            # NB: processEvents() does not dispatch DeferredDelete events
+            QCoreApplication.sendPostedEvents(None, QEvent.Type.DeferredDelete)
+            qapp.processEvents()
+            gc.collect()
+            qapp.processEvents()