+++ This bug was initially created as a clone of Bug #1819916 +++
We made a fix for bug 1819916 but it's not ideal. There's some discussion in that bug and I put some things I discovered in comment 25.

Let's see if we can make a better fix.

Removes the gist of https://hg.mozilla.org/comm-central/rev/10277c24518637c05ec21f303d7fad5a324e3c0f
and implements something similar (and some cleaning up) in the problematic IMAP code.

Gene, does this handle the problematic messages you had in the other bug okay?

Applied the patch and still looks good on my large test messages while also showing the original reporter's smaller test message without missing stuff. So looks like a good fix.

Off topic, but when I open one of my large messages (about 100Mb) there is no progress or progress bar graph in the status bar. Initial load via imap takes a while and there is no progress feedback while waiting for the message and attachments to completely appear like there was with 102. Has this been intentionally changed or is it a bug?

Yeah, what happens when messages load slowly sure isn't great. It's not intentional, but it's not been a priority either. I can't think of a specific bug number for it.

OT;
I'm not so concerned about the slow loading (kind of expect it since these are huge messages even when loading from offline store or cache). What I'm saying is the status bar progress feedback is missing. I'll do a search and see if it's been reported and, if not, file a new bug.
Edit: Looks like someone reported it here: Bug 1832149

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/b3523ca25ec5
Avoid closing IMAP mock channels until OnStopRequest completes. r=gds

This fix didn't really do the job, and I don't want to clone the bug again for the next patch.

pretty sure i only see this on messages with attachments, like this one. TB117b6. Attachment is a PDF if that matters.
Doesn't matter if you try View -> Headers -> All or Normal.

I see the randomly missing headers in preview when testing with message in comment 8. It occurs both with and without mbox offline store. Didn't try it with maildir. Tested just with locally built daily 118.0a1.

Observation that may or may not be relevant: When I open very large emails (e.g., 100Mb) from storage (mbox or disk cache) the body appears first and then the header. With older versions (110 or earlier, pre-SN) the header appeared first and then then body. It's not noticeable with small or average size emails.