Found a bug that I've been aware of for some time, but have been unable to pin it down...

If you 'edit' a post, replacing test with something different, and the add a mark-up to the text, such as making it bold, the old text that was replaced reappears.
Seems to cause different issue if you select just the new text to mark-up, or include some of the original text.
I've seen something similar when adding URL links to text too.

Hope that help.

Steve

I sometimes notice a similar behavior, but thought it was Firefox issue, hard for me to recreate. I thought adding selektion=""; would fix it but it still appears.

I don't use Firefox, so its not that.
I have seen this with Chrome and Edge.

Steve

Thanks for reports.

I know such bug exists, I've met it sometimes on my own, too. Sometimes but not always, mostly on Windows desktop (not Android mobile) and could not re-create it so far. For example when working out the post I highlight the text and click the Bold button, but obviously it's replaced with another snippet and not the selection. This appears only once, and not always, and if I revert everything and repeat this again, it works normally.

Since I've not changed anything in javascipt selection functions, I could only suppose something was changed in all browser models, maybe it's because of some bug in Gecko core, which is nowadays in the base of almost each browser. But I'm not sure and whatever I could suggest — let's watch it more closely and try to find a way which absolutely recreates this bug on some step.

Btw Editing and Posting processes use the same form and the same Javascript functions, so this bug is just about deeply unknown in the browser model.

So the most likely problem is the paste_strinL function?

I couldn't be sure on that...

If it was possible to recreate every time, it would be easier... I suspect something happens after highlighting text in the post form but, most likely, some special text already must be present in the form (I suspect some BB code) and there should be some text deletion in the form; with just regular text I don't feel I've ever encountered it.

It's surely JavaScript issue like reported initially and it's not about a bug in PHP script, if you are about that. It's only about client-side. Something wrong with the selection algorithm, it sometimes copies to buffer (and pastes back) not exactly the same phrase which was selected in a posting form. That's about it.