mailto: unable to correctly parse some URIs (e.g. lore.kernel.org)

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There are PRs with fixes to some mailto parsing issues but this one seems to have escaped them! Discovered whilst trying to reply via mailto link in a Linux kernel mailing-list thread on lore.kernel.org; e.g:

https://lore.kernel.org/all/ZnCviUrk5iQGrE4x@codewreck.org/#t

In this case the link provided is:

<a
href="mailto:asmadeus@codewreck.org?In-Reply-To=%3CZnCviUrk5iQGrE4x@codewreck.org%3E&#38;Cc=andrea.righi%40canonical.com%2Cbluca%40debian.org%2Cckk%40debian.org%2Cdhowells%40redhat.com%2Cema%40debian.org%2Cericvh%40kernel.org%2Clinux-afs%40lists.infradead.org%2Clinux-cifs%40vger.kernel.org%2Clinux-fsdevel%40vger.kernel.org%2Clinux-kernel%40vger.kernel.org%2Clinux-mm%40kvack.org%2Clinux%40iam.tj%2Cv9fs%40lists.linux.dev&#38;Subject=Re%3A%20%5BPATCH%20v5%2040%2F40%5D%209p%3A%20Use%20netfslib%20read%2Fwrite_iter">mailto: link</a>

Using Firefox v128.0a1 amd64 on Debian GNU/Linux 12 Bookworm, this takes me via a "Switch" account intermediary page to:

https://mail.proton.me/u/1/inbox#mailto=mailto%3Aasmadeus%40codewreck.org%3FIn-Reply-To%3D%253CZnCviUrk5iQGrE4x%40codewreck.org%253E%26%2338%3BCc%3Dandrea.righi%2540canonical.com%252Cbluca%2540debian.org%252Cckk%2540debian.org%252Cdhowells%2540redhat.com%252Cema%2540debian.org%252Cericvh%2540kernel.org%252Clinux-afs%2540lists.infradead.org%252Clinux-cifs%2540vger.kernel.org%252Clinux-fsdevel%2540vger.kernel.org%252Clinux-kernel%2540vger.kernel.org%252Clinux-mm%2540kvack.org%252Clinux%2540iam.tj%252Cv9fs%2540lists.linux.dev%26%2338%3BSubject%3DRe%253A%2520%255BPATCH%2520v5%252040%252F40%255D%25209p%253A%2520Use%2520netfslib%2520read%252Fwrite_iter

but the resulting composer panel only has the To: field completed.

In particular I was expecting the Subject: and In-Reply-To: fields since that is how replies are threaded on lore.kernel.org but the BCc and Cc are also missing.

Start by reproducing the supplied lore.kernel.org mailto link in Proton Mail and inspect how its URI parameters reach the composer. Done means the composer populates the In-Reply-To, Cc, Bcc, and Subject fields as well as the recipient.