I've ran into this today, the popup doesn't appear when creating a new page that did not exist before!

This matches with what Rummana wrote when reporting this initially:

Did not receive the pop-up that I got auto-subscribed after posting a new discussion on someone else's page for the first time after enabling automatic topic subscription.

…although "for the first time" was ambiguous, I assumed then that you meant that it was the first topic you posted anywhere after enabling the pref,

Now I'm hoping you meant that it was the first topic on a page that didn't exist, and that the issue I've seen is the same ;)

Change 739820 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/extensions/DiscussionTools@master] Ensure highlighted comment is defined before we try to show auto topic sub popup next to it

https://gerrit.wikimedia.org/r/739820

The bug was caused by a conflict between the patches for mobile support and the patches for auto topic subscriptions. We merged the mobile stuff first, and it changed the order of some operations after creating new talk page to work better on the mobile site, but the auto topic subscription stuff relied on the previous order and I didn't notice the issue before we merged that.

Change 739820 merged by jenkins-bot:

[mediawiki/extensions/DiscussionTools@master] Ensure highlighted comment is defined before we try to show auto topic sub popup next to it

https://gerrit.wikimedia.org/r/739820

In T295945#7513762, @matmarex wrote:

I've ran into this today, the popup doesn't appear when creating a new page that did not exist before!

This matches with what Rummana wrote when reporting this initially:

Did not receive the pop-up that I got auto-subscribed after posting a new discussion on someone else's page for the first time after enabling automatic topic subscription.

…although "for the first time" was ambiguous, I assumed then that you meant that it was the first topic you posted anywhere after enabling the pref,

Now I'm hoping you meant that it was the first topic on a page that didn't exist, and that the issue I've seen is the same ;)

@matmarex: Ah, sorry for being ambiguous! You are right, it was the first topic on that talk page.

And it's fixed now. so thanks for that! :)

In T295945#7513815, @matmarex wrote:

The bug was caused by a conflict between the patches for mobile support and the patches for auto topic subscriptions. We merged the mobile stuff first, and it changed the order of some operations after creating new talk page to work better on the mobile site, but the auto topic subscription stuff relied on the previous order and I didn't notice the issue before we merged that.

Thanks for the context.