Bugzilla

The exception is thrown when [abortPrompt](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/toolkit/components/prompts/src/CommonDialog.jsm#327) is called on CommonDialog.jsm. This happens in a couple cases:
- "unload" and "TabClose" events [here](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/toolkit/components/prompts/content/tabprompts.jsm#271)
- Switching away from a tab with an active beforeunload prompt [here](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/browser/base/content/tabbrowser.js#1189)
- The PromptParent window actor calls [abortPrompt](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/browser/actors/PromptParent.jsm#110,124) on destroy and when the content fires `pagehide`

There could be a race condition with how we handle `pagehide` events. Since they are fired in the content process, in the new implementation they are sent to the parent via an [async actor message](https://searchfox.org/mozilla-central/search?q=Prompt%3AOnPageHide&path=). The old implementation handled this with an [event listener in Prompter.jsm](https://searchfox.org/mozilla-central/diff/3075dbd453f011aaf378bcac6b2700dccfcf814c/toolkit/components/prompts/src/Prompter.jsm#517) in the content process.

I've seen a similar issue in another test: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=300037356&repo=try&lineNumber=14207

The exception is thrown when [abortPrompt](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/toolkit/components/prompts/src/CommonDialog.jsm#327) is called on CommonDialog.jsm. This happens in a couple cases:
- "unload" and "TabClose" events [here](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/toolkit/components/prompts/content/tabprompts.jsm#271)
- Switching away from a tab with an active beforeunload prompt [here](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/browser/base/content/tabbrowser.js#1189)
- The PromptParent window actor calls [abortPrompt](https://searchfox.org/mozilla-central/rev/7fd1c1c34923ece7ad8c822bee062dd0491d64dc/browser/actors/PromptParent.jsm#110,124) on destroy and when the content fires `pagehide`

There could be a race condition with how we handle `pagehide` events. Since they are fired in the content process, in the new implementation they are sent to the parent via an [async actor message](https://searchfox.org/mozilla-central/search?q=Prompt%3AOnPageHide&path=). The old implementation handled this with an [event listener in Prompter.jsm](https://searchfox.org/mozilla-central/diff/3075dbd453f011aaf378bcac6b2700dccfcf814c/toolkit/components/prompts/src/Prompter.jsm#517) in the content process. So given that browser (+ JSWindowActor pair) is reused it could be side effects from a preceding test.

I've seen a similar issue in another test: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=300037356&repo=try&lineNumber=14207