In a recent Xcode update (maybe 26.5, maybe all 26 versions), Xcode started inserting the closing quote, parenthesis, or bracket. It happens like this. I type, say, an opening double quote ("). Then as soon as I type another character, it adds another double quote just ahead of the insertion point.
Now, I have everything in the Settings->Editing->Completion section turned off. I also have virtually everything in the Indentation section turned off.
Is this just a bug? I've reported it (FB22693037). But I'm surprised it wasn't caught by internal testing.
I can't express how much I hate this behavior.