fix: chrome crashing on closing dev tools (fix #2103)

[tiagoroldao]

Description

Fixes #2103
When closing devtools, an infinite loop is triggered by the line:
https://github.com/vuejs/devtools/blob/963ff7fe1470aa5b2a93a8a8afadc57fa3c38676/packages/shell-chrome/src/backend.js#L47

After closing devtools, the window.postMessage call in backend.js triggers the event listener in the same file, and as the reaction to a message is to postMessage back, an infinite loop is triggered this way.

Additional context

To the best of my knowledge:

• this should not happen, and is likely a Chromium issue This is an expected behavior, likely not triggered on initial setup due to a race condition
• this only happens when devtools are closed (the logic that filters out self from the triggers for window.on('message') breaks down only in this case)
• packages/shell-chrome/src/backend.js is the only file in shell-chrome where a window.postMessage is triggered as a fallback on a window.on('message') listener, so no other places have this issue
  • shell-firefox uses the same (duplicated) code, so this change may be warranted there - at worse it will do nothing, and will prevent any situation where, due to changes in event order, this bug would manifest there

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).