Trace window focus, blur and visibility in pen input

From the development repository at 0304046.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_end_to_end.py

@@ -781,7 +781,7 @@ class EndToEndTest(unittest.TestCase):
         # and a release of the pen, after the browser's name.
         shown = firefox.run("return document.getElementById('event_log').textContent;")
         self.assertRegex(shown, r"^\d\d:\d\d:\d\d\.\d\d\d Browser: Mozilla")
-        self.assertRegex(shown, r"\n\S+ press pen \d+ buttons 1 pressure \d\.\d\d, pen\n")
+        self.assertRegex(shown, r"\n\S+ press pen \d+ buttons 1 pressure \d\.\d\d \((?:in front|behind)\), pen\n")
         self.assertRegex(shown, r"\n\S+ pointerup pen \d+, \d+ points")
         firefox.run("document.getElementById('session_close').click();")
         self.assertFalse(firefox.run("return document.getElementById('session_dialog').open;"))