OAuth/Popup broken - opens external browser

[tennox]

Describe the bug
Trying to implement OAuth flow, but MetaMask opens the URL in a different browser and thus there is no connection to send back the authorization code via e.g. window.postMessage

To Reproduce
I tried making a minimal sandbox example:
https://codesandbox.io/s/popup-test-jmbgp

1. Open on PC to start sandbox
2. open sandbox app URL (e.g. https://jmbgp.csb.app/) in Metamask android app
3. See that MetaMask opens popup in extra browser outside of metamask app, and the postMessage calls fail

Expected behavior
On most other browsers this works and is often used for e.g. OAuth flows

Smartphone (please complete the following information):

• Device: OnePlus 6T
• OS: Android 10.3
• App Version: v2.4.0

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to be added after bug submission by internal support / PM
Severity

• How critical is the impact of this bug on a user?
• Add stats if available on % of customers impacted
• Is this visible to all users?
• Is this tech debt?

[github-actions]

This issue was closed because it has been stalled for 7 days with no activity. If you feel this was closed in error please reopen and provide evidence on the current production app. Thank you for your contributions.

[tennox]

This issue is still valid and reproducible via the link in the description