I know it's more nuanced than this, but generally (in the US, at least) holders of a trademark need to defend the mark at the peril of "abandonment" or the mark becoming "generic". I expected to see something about this in the post.
The Notepad++ guy wasn't an attack dog here. I'm 100% behind his reasonable position. Just saying, you're not legally obligated to say "no, no one else in the entire world can use it". It's more that you have to say "no one else can use it without my permission." Also just saying, if you want to use someone else's trademark, it's a really, really bad idea to start the conversation by using it without their permission and thereby requiring them to decide right then and there whether they'll allow you to. It's kind of like asking to borrow someone's car versus taking it first and then asking if that was OK.
I think it's a case of where a lot of people don't have experience with trademark licensing.
Back in the 90s I worked for a small company that was in various "partner" programs with hardware and software companies (Microsoft, Intel, Citrix, etc). Each "partnership" agreement came with trademark licensing documentation and very, very specific usage requirements for using the trademarked name, logos, etc. With at least one of the companies we had to get ad copy approved for compliance with our license.
The single biggest thing is that you have to stop people from using it without permission. If that means you stop them by granting that permission, then so be it.
I’m not sure if Google has ever done similar but people use google as a verb to mean web search and I’m not sure if they worried about losing brand protection or though of it as an advantage in acquiring users.
Is there a clear source for this mechanism?
There isn't a clear source (at least that I'm aware of) since this is handled by the legal system with a lot of nuance on a case by case basis. It is very reasonable to be proactive in trademark defense but if you aren't courts may still side with you if the establishment of usage was clear.
[0] https://old.reddit.com/r/nintendo/comments/5m9grz/theres_no_...
Incidentally, this is why Richard Stallman objects to the term intellectual property. It bundles together three very different areas of property rights (copyright, trademark, and patents) and treats them as sort of a single entity, even though they're really very different, both in their reason for existing and in their mechanisms.
People believe so many dumb things about copyright, trademark, patents, and trade secrets. For example: You don't legally need to use the various symbols for trademarks and copyrights you don't own. Unless there's a contract in force saying you have to, there's no Symbol Police gonna rappel from the skylights and break your keyboard if you say you used a Xerox without the nifty ™ symbol.
Another thing I've seen is the apparent notion that you can "renew" a copyright. Nope, not for a long time now: You get the full term up front with no special action, and once it's done it's gone, unless the law is actually changed in the meantime. Disney didn't "renew" the copyright on "Steamboat Willie" and the dumb live-action remakes aren't being done to "renew" anything, they're just some executive having a brain fart.
Like "Steamboat Willie", which I believe was mentioned here recently.
This issue with Notepad++ isn't even really about trademark, even though that would be the legal means to enforce it. It's more about Criteria #4 of the opensource definition "Integrity of The Author’s Source Code" Which is there so you can't give the original project a bad name while forking it. https://opensource.org/osd