This test is flaky failing throughout the visible history. The oldest failure with a diff that I'm able to locate is r254332.
@@ -9,5 +9,5 @@
IFrame display height = 500
Mouse moved to (28, 466)
-PASS IFrame received wheel events.
+FAIL IFrame did not receive wheel events.
I have marked this test as failing while this issue is investigated.
No longer flakey. Expectation removed in r258701.
*** This bug has been marked as a duplicate of bug 197819 ***