A few emails I receive will cause claws-mail to hang in a CPU spin when
attempting to preview it (by selecting it in the message list for
preview in the third pane). This CPU spin consumes 100% of one core,
with nothing visible to strace. The program does not respond to regular
SIGINT/TERM and requires SIGKILL to quit it.

These cases are entirely repeatable, and all occur on github issue
notifications from one particular author. Most notifications from that
author do not hang, and work as expected. But occasionally one
notification will cause a hang, and I have noticed the coincidence that
it is always the same author.

I shall attach a file directly from my Maildir containing such a case.

Attached is the problematic email, extracted from my Maildir/IMAP
server.

It is a github notification of:
https://github.com/Perl/perl5/issues/18624#issuecomment-816733358

control: tags -1 moreinfo

Hi Paul,

Since is not specified I've tried to view your message without plugins, with
litehtml plugin and with dillo plugin. Didn't find any problem viewing the
message in the three cases, so I guess is something particular on your
configuration or machine. To narrow a bit the problem, which other plugins
have you loaded? Does removing any of them (try one at a time) improve the
situation?

Also, since it's reproducible in your machine, and before experimenting with
the plugin list, can you run claws-mail with --debug flag in a terminal and
see if something interesting is printed?

thanks in advance!