Thunderbird displays the dialog for default application after each startup

Since Thunderbird 87 Beta 1 a dialog is displayed that wants to set Thunderbird as default for various purposes (like email or calendar). This dialog is displayed even though Thunderbird is already set as the default email application. In my case, I did not check the checkbox for the default calendar application.

Steps to reproduce:

1. Make sure that the checkbox "On startup, check if Thunderbird is registered as the default application" is checked in the preferences and Thunderbird is not set the default calendar application.
2. Restart Thunderbird.

Result: Thunderbird asks if you want Thunderbird to be the default application for Calendar every time you start the application, even though Thunderbird is already the default application for email.

Expected: Thunderbird should only display this dialog if none of these options are enabled. It may be that a user wants to use Thunderbird for email but not for calendars. However, he wants Thunderbird to check at each startup whether Thunderbird is still the default application for email.

you are on windows 10?

(In reply to Wayne Mery (:wsmwk) from comment #1)

you are on windows 10?

Yes, latest version.

We're doing the last planned beta today, and no patch in sight, so this won't get fixed in 87.
It would be useful to identify the patch that regressed this.

Hopefully Alice can help finding the regression.

fwiw, the "check now" button within Options shows both the email and calendar checkboxes unchecked, indicating it's whatever code is checking the current OS integration state that's failing to identify the Thunderbird is the default app.

It's a pretty significant visible annoyance imo (scope of affected users is unclear though)

Regression window:
https://hg.mozilla.org/comm-central/pushloghtml?fromchange=bc12f43853bade510d9f67c577380040ceb0836c&tochange=5950078d065f23e6dfaccb0ca92b0f964cf6dc6d
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=791ad465a0c8344f38d54f0a226860fc26cfe540&tochange=d1b7430e5ebbe7a782c76959e2c8e1d87a5107c6

Thanks, seems mid support on windows needs more work. Will work on it.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/b711c5f22343
Fix mid: support on Windows. r=mkmelin

Comment on attachment 9209036 [details]
Bug 1696218 - Fix mid: support on Windows. r=mkmelin

[Approval Request Comment]
Regression caused by (bug #): bug 264270
User impact if declined: Because mid: support was not registered correctly on window, set TB as default mail app doesn't work
Testing completed (on c-c, etc.): c-c
Risk to taking this patch (and alternatives if risky): low

Comment on attachment 9209036 [details]
Bug 1696218 - Fix mid: support on Windows. r=mkmelin

[Triage Comment]
Approved for beta