Thunderbird spellcheck does not work sometimes when replying to message

I have also tried to inspect this behavior. As you have provided the detailed information, it is most likely a bug, which will be hard to trace and fix, because this happens occasionally. It is most likely connected to the changes how Thunderbird displays the message.

What needs to be done within Mozilla Thunderbird is to reset any formatting and then reactivate it again.

To answer your question: There is no simple way to patch this behavior.

I can only offer a workaround where you first press alt + o and then shift + s - this deactivates it and use the same combination to reactivate it.
Other option would be to have a custom shortcut for such functionality.

This sounds similar to existing Thunderbird bugs:

Inline spell checking randomly fails

[ . . . ]

The actions that cause the inline spelling to fail (i.e., the red underlining to disappear) are normal editing. For example in one instance, I noticed that backspacing over a word caused the red underlining to disappear from a previous spelling error (a couple lines removed)).

Bugzilla-933813

What I found is that any new message checks the spelling correctly as you type (underlines). Reply to any message and the check spelling as you type does not work (no underlines). Setting mail.compose.max_recycled_windows to 0 works with both new messages and when you reply. I only have the english dictionary installed.

Launchpad-Bug-1229172

I can't reproduce with TB18, html or plain text compose, and the mozillazine steps

1. Open a new email message.

2. Type 'In the middle of the day'

3. Backspace until you only have 'In the mi'

4. Then retype the rest so you end up with 'In the middle of the day' again.

However, some say it is intermittent

Bugzilla-790475

Even if it's not the same bug, you can sometimes get ideas from other bug reports.

Dealing with intermittent or un-reproducible bugs can be frustrating. I've been there many a time, and it's not fun. Here's some things you could try:

• Record a screen-cast of the bug when it occurs.

  Try to avoid including any personal or identifying details. This makes it a lot easier to show what's going on; you can right-click and show you have "Check Spelling" enabled, as well as what dictionaries are installed.

• Does manual spell checking with Ctrl-Shift-P work?

  Based on your comments, it sounds like it is, and just the highlighting is broken. This is something you could demonstrate in the screen-cast.

• Is it reproducible for a particular message?

  That is, can you hit reply, observe the bug, then close and re-open Thunderbird, hit reply on the same message, and get the same bug again?

• If you cut and re-paste the text, does the spellchecking start working again?

  In some bug reports, it seems like this makes a difference.

• Does HTML or text composition matter?

  If you hold down shift before clicking the "Reply" button, it will toggle between HTML and text composition.

• Try setting mail.compose.max_recycled_windows to 0.

  It seems like it helped in other bug reports.

• If you run Thunderbird from the console, do you see any relevant error messages in the output?

  This is a bit of a long shot, but it might be worth a try.

• Do you still observe the behavior if you run in safe mode, i.e. Help -> "Restart with Addons Disabled"?

  I'm guessing yes, but it's a good idea to test this since it helps establish it's an issue with the core Thunderbird code, not some ancillary extension.

• Do you still observe the behavior if you make a new profile?

  This can be time-consuming, but it's a very useful data point: it means that it's not a quirk of your profile or settings.

You can also take a peek at the source code. Sometimes just reading the comments can be helpful. Here's some relevant parts for Debian buster:

Link-1, link-2, link-3

If you think it's worth submitting a bug, you can do so either via email with Debian or directly to Thunderbird here.

Thunderbird spellcheck have some limitation/settings, especially when we write/reply-to a long email it can get disabled, this is because of the setting:

extensions.spellcheck.inline.max-misspellings  

default value is 500 if we increase that to 10000 or a higher value the spellcheck won't get disabled because of the email length.