Problem: TestWebDriverSessionHistory flakes on slow (Sanitizer) CI
runners. The subtest checking that cancellation of a script-initiated
cross-site “back” by a beforeunload handler intermittently failed the
assertion “Expected beforeunload to cancel script-initiated cross-site
history.back(), got [url_b, 0]”. scriptBeforeUnloadCount was 0, not 1.
Cause: history.back() appends its traversal to the traversable’s session
history queue and returns immediately. So the beforeunload prompt to
unload runs async. The test read scriptBeforeUnloadCount in a separate
execute_script right after — with no wait. So on a slow runner, the read
beats the dispatch, and sees 0. Every other script-initiated navigation
in the test waits for its target document to load — and the “refresh”
beforeunload check already polls. But the canceled “back” loads no
target document — and this one waited for nothing.
Fix: Poll with wait_for_script_result until the page is still on /b and
window.scriptBeforeUnloadCount has incremented, before asserting the
final state — mirroring the “refresh” beforeunload check.
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