Is there any way to disable the cursed "In selection" checkbox forever?
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I rarely use the “In selection” checkbox and Notepad++ constantly checks it every time I select even a single character and it’s driving me insane. If I wanted to use it I would manually check it. Is there any way to stop this annoying behavior?
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Settings > Preferences > Searching > Minimum Size for Auto-Checking “in selection”:

It defaults to 1024, meaning you need to select at least 1024 characters in order to get it to checkmark the “in selection” box with the default setting. If you changed it to a lower number (like “1”), that would explain why it always auto-checks that box.
And if you really want never, instead of >=1024, then set that to
0, as it says when you hover your mouse cursor over the(?)and in the user manual’s section on the “Searching” preferences.