I was trying to update my thread recently and I eventually ended up losing a wall of texts from my previous post because everything looks normal after pasting but the half of the post was gone after hitting the post bottom

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You don't lose your post content, that is only not displayed well.
You can click on your post, edit and see the full post content.
There’s no extra allocation, I think it’s the way the backend handles the storage in DB .. not sure but it probably using
TEXT because it’s the one that stores upto 64KiB, unless there’s just another way they implemented that 65535 bytes.
Once you post the post get cutoff.. it can be your existing post content depending on how you edit. E.g if you paste new content at the top or mid and you hit the limit, the existing contents at the bottom will be cutoff with its calculation. Clicking on edit won’t bring back anything unless you already preview it before posting, you can copy the entire BBCODE content from your drafts..
Edit: I’m not sure the draft won’t cut it off as well
If you are still worrying about content loss, you can use a sort of document composing or note tools to compose post, save the content there before copying it to your forum post.
What made me lost mine was because I copied and pasted new content in the code block which was mid so I lost part of the code block and entirely the last part of my content and of course, I didn’t preview it…
AFAIK it's 64,000 characters. I tested it recently and it hasn't changed.
Posts over 65535 bytes get cut off at 65535 bytes. If that makes the BBcode invalid in certain ways, then you get the INVALID BBCODE error.
Also, note that a byte is different from a character. Because SMF stores "special" characters in a particularly space-inefficient way, each Russian character uses about 6 bytes, and each line break also uses 6 bytes.
Thanks for this.. from what I learnt yesterday, ASCII char is usually 1byte so 64000 chars is probably around that region..not sure where the remaining space goes though, probably reserved or something.. But you will fill the space up faster with this : “
Also, note that a byte is different from a character. Because SMF stores "special" characters in a particularly space-inefficient way, each Russian character uses about 6 bytes, and each line break also uses 6 bytes.”