mailto: links using percent encoding for UTF-8 encoding, avoiding RFC 2047 encoding, do not work

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App version

5.807

Where did you get the app from?

F-Droid

Android version

LineageOS

Device model

No response

Steps to reproduce

I click on the html code

<a href="mailto:?to=%D0%90%D0%BD%D0%B0%D1%81%D1%82%D0%B0%D1%81%D0%B8%D1%8F%20%3C1%40example.org%3E">zzz</a>

K9 mail opens, and contains empty TO: field.

If I take the decoded string (Анастасия), first RFC2047 ecnode it, then percent encode it, and click the link, K9 fills the TO: field. My reading of RFC6068 Examples Using UTF-8-Based Percent-Encoding is that both are equivalent:

• mailto:user@example.org?subject=caf%C3%A9
• mailto:user@example.org?subject=%3D%3Futf-8%3FQ%3Fcaf%3DC3%3DA9%3F%3D

So it is possible to skip the RFC2047 encoding and use only percent encoding on the UTF-8 data.

Expected behavior

I click on the html code

<a href="mailto:?to=%D0%90%D0%BD%D0%B0%D1%81%D1%82%D0%B0%D1%81%D0%B8%D1%8F%20%3C1%40example.org%3E">zzz</a>

K9 opens and shows empty TO: field.

Actual behavior

K9 mail shall fill To: Анастасия <1@example.org>

Logs

No response

Start by reproducing the supplied HTML mailto link on the reported app and Android setup, then trace the mailto handling entry point for percent-decoded UTF-8 recipient data. Done means the link populates the To field with “Анастасия 1@example.org” without requiring RFC 2047 encoding.