Browser extensions might attach event listeners to the DOM so the safest way to run this test is by running Chrome from command line:
google-chrome \
--enable-precise-memory-info \
--disable-extensions \
--disable-plugins \
--incognito \
http://localhost:8125
The above will run Chrome without extensions or plugins in incognito mode and open manual tests page.
Close any running Chrome instance beforehand because otherwise it will open a tab in currently opened Chrome (look for "Opening in existing browser session." in command line).