I guess it depends on whether it's likely to confuse people?
> Is Notepad++ available for Mac?
> Yes. Notepad++ is now natively available for macOS as a free download.
That's over the line. This isn't a few tweaks to get it to compile on a Mac, but a wholesale rewrites of big chunks of it. It's a fork of Notepad++, but it's not the Notepad++.
Saying “Awesome Notepad, a Notepad++ fork” as the tagline for a product would then be using Notepad++’s trademark for Awesome Notepads branding.
If it’s just mentioned in the documents then it because a question for the courts to decide if that’s sufficient use in advertising material. And what will likely usually happen is an undisclosed settlement.
But a lot of this depends on how it’s used, how litigious the trademark holder is, and what jurisdiction this is happening in.