Bugzilla

So I ran 
```
./mach wpt testing/web-platform/tests/mathml
./mach wpt testing/web-platform/tests/mathml --setpref fission.autostart=true --setpref gfx.webrender.all=true
```
and got the following results respectively:

**Without Fission**
```
web-platform-test
~~~~~~~~~~~~~~~~~
Ran 2164 checks (1 asserts, 1981 subtests, 182 tests)
Expected results: 2160
Unexpected results: 4
  test: 4 (4 fail)
```

**With Fission**
```
web-platform-test
~~~~~~~~~~~~~~~~~
Ran 2164 checks (1 asserts, 1981 subtests, 182 tests)
Expected results: 2161
Unexpected results: 3
  test: 3 (3 fail)
```

I'm assuming that fact that more tests failed without fission is probably just an intermittent test failure. I was hoping for a test case that passes without fission and does not pass with fission so that I can start looking into amending this. I didn't find one here, unfortunately. 

Does anyone have an idea for a test case that would explicitly trigger something wrong with `nsIDocShellTreeItem` in this particular file?

So I ran 
```
./mach wpt testing/web-platform/tests/mathml
./mach wpt testing/web-platform/tests/mathml --enable-webrender --enable-fission
```
and got the following results respectively:

**Without Fission**
```
web-platform-test
~~~~~~~~~~~~~~~~~
Ran 2164 checks (1 asserts, 1981 subtests, 182 tests)
Expected results: 2160
Unexpected results: 4
  test: 4 (4 fail)
```

**With Fission**
```
web-platform-test
~~~~~~~~~~~~~~~~~
Ran 2162 checks (1 asserts, 1974 subtests, 187 tests)
Expected results: 2162
Unexpected results: 0
```

I'm assuming that fact that more tests failed without fission is probably just intermittent test failures. I was hoping for a test case that passes without fission and does not pass with fission so that I can start looking into amending this. I didn't find one here, unfortunately. 

Does anyone have an idea for a test case that would explicitly trigger something wrong with `nsIDocShellTreeItem` in this particular file?