Re: Is there any way to disable it, I remember I had no problem with this before, only after I remove the app and installed it again.
It's been like that forever and there never was (and is not) a setting to turn it off.
It's "standard" - as much as this can be called "standard".
There is a number of different prefixes that mail apps use.
Some use Re: and Fwd: - these seem most common, so we used that.
I've never seen "FW:" until now.
We don't add a prefix if it's already there. But in your case there is no match so Aqua's "Fwd:" gets added to your "FW:".
Some "collapse" repeating prefixes like this:
"Fwd:" + "Fwd: Re: Fwd: Fwd:" -> becomes "Fwd[2]: Re: Fwd:"
Some don't collapse.
Some use the user's language (non-English), in Italy the prefixes for replies / forwards are just "R:" and "I:" (one letter!) in Germany they are sometimes "Antwort" (complete word!).
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As far as "change of subject messing up message threading" - this is up to the receiving app doing the thread collation. Ours handles all the cases above (both "Re:" and "Re[3]:" and special cases for Italian and German).
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I will add a special case in our code.
Now:
"Before adding Fwd: - check if the subject already starts with Fwd: and if so don't add Fwd:"
Will become:
"Before adding Fwd: - check if subject already starts with Fwd: or FW: and if so don't add Fwd:".