IE & Edge crash with "Permission denied" on the `contents() for <frame />` test

[mgol]

Description

IE 11 as well as Edge (even 17-18) often crash on the traversing: contents() for <frame /> test with the "Permission denied" error. IE crashes way often than Edge. This is probably related to my Sizzle removal PR (#4395) but I wasn't able to pinpoint the exact reason so far. The error points to line 174 in src/selector.js. An example failing run: http://swarm.jquery.org/result/3337866.

It's hard to debug as it's clearly a race condition. If I run this test in isolation, it always passes.

I think I've seen a similar failure on the 3.4-stable branch as well but it was happening way less often.

We should figure this out before the 4.0.0 release.

Link to test case

N/A, it's a core test already in the repo.

[mgol]

Unfortunately, while in the vast majority of cases the test case fails in IE, when you run that particular test in isolation it almost never fails. This makes debugging tricky.

[mgol]

Some conclusions from my investigation:

1. The error is triggered by this line:

   var contents = container.find( "#test-frame" ).contents();

2. This is not the only test failing with "Permission denied". I initially thought this one failure triggers more of them sometimes but commenting out that one test results in frequent failures in some other tests, e.g. the traversing: sort direction test. The stack trace doesn't point to anything that could help.

[mgol]

I found the cuplrit. IE/Edge sometimes crash when comparing documents between frames... but only if one of the strict equality operators (=== or !==) is used. Shallow equivalents (== or !=) work fine... PR incoming.

This means the issue might as well exist on 3.x as well, maybe it just triggers the condition less often. I'm changing the milestone to 3.4.2 & I'll submit a PR to Sizzle as well.

[mgol]

jQuery PR (for jQuery 4.0): #4471.