Enigmail appears to be signing messages before thunderbird applies a line-wrap. To replicate create an email with a long line of text.
That's why Enigmail applies line wrapping before signing, unless you disabled this in Enigmail ...
The Wrap HTML before send setting in enigmail was on. The problem still occurs when this is set.
I've tried to enable some advanced wrap features that diverge from the defaults in hopes they fix the problem. I set "plain_text.wrap_long_lines;true" and this did not help. The default settings of "mail.wrap_long_lines;true" "extensions.enigmail.wrapHtmlBeforeSend;true" were already set and also did not help. Any suggestions are welcome.
If under the account settings I turn off HTML formatting and change this to plain text the thunderbird editor handles line wraps dynamically as you type. This seems to be the only way to solve the issue.
But this definitely indicates some sort of bug between enigmail and thunderbirds conversion from HTML to plain text formatting.
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