I'm not familiar with how the NTP logic changes the title.  treib@, do you know someone on the NTP team that can look at this, or at least point out where the bug is?

I'm not familiar with this either, but I'm the only one actively working on the desktop NTP. I'll take a look at some point.

It's been a long time since I looked at this in Chrome. IIRC, SearchTabHelper listens for navigation changes and sets the title (assuming the NTP hasn't overridden it server side). See:
https://cs.chromium.org/search/?q=IDS_NEW_TAB_TITLE+package:%5Echromium$+f:search_tab_helper.cc&type=cs

I suspect replaceState is resulting in a type of navigation change that isn't handled here.  So one possible fix might be to add another listener.

(Of course, there might be a better, less brittle way of doing this as well.)

See also https://codereview.chromium.org/78443005/ where this approach was first introduced (obviously, it's gotten somewhat more complicated since then :)).

So, maybe I'm dumb... but why can't the NTP content have <title>New Tab</title>?  Yes, that would let custom new tabs overridden by some other default search provider call themselves something else; I'm not sure that's so bad.

Search providers can already do that. The "API" here is: the server NTP can set a title and it will get used, or it can omit a title, and Chrome uses its standard title for NTPs.

The downside for the server picking the title is that the server can't use Chrome's translations/string resources and so has to re-create all of this (which languages to support, etc) and so things will differ slightly. If Chrome ever changes what it wants to title the NTP, this would need to be coordinated as well.  This is also why this wasn't an option for a quick fix here: we don't have translations available right now on the server.

But: it seems like having Chrome set the title when unset has turned out to be surprisingly convoluted (e.g. compare the original version in https://codereview.chromium.org/78443005/ to what we have now). So it's not unreasonable now to just give up on this and have the server set the title.

Longer-term, I'm also working towards removing the remote NTP entirely, see bug 583289. That would avoid this problem completely.

Hm, on second though, the local NTP will not actually make this obsolete, as we'll still have third-party remote NTPs.