I don’t think watchlist and topic subscription together (i.e. subscribing a topic and watching the page it’s on) make much sense in most people’s workflows. So if any, checking “Watch this page” should disable topic subscriptions. But it’s probably even less intuitive than not doing anything, so I suggest leaving the two features orthogonal. Maybe the reply/new discussion tools could have another affordance that controls topic subscription, placed outside of the “Advanced” section. It would make the UI more cluttered, but it’d have several benefits:

• It’d resolve this task. If there are two, independent checkboxes for the two features, it’s clear that the features are also independent.
• It’d make the topic subscription feature more discoverable. Since the user already focuses on the comment interface, it’s more likely that they notice this checkbox than that they notice the subscribe link next to the topic title.
• It’d make the automatic topic subscription behavior more predictable. If posting the comment subscribes the user or keeps them subscribed, the checkbox is checked by default, if posting the comment keeps them unsubscribed, the checkbox is unchecked by default. (If they uncheck the default-checked checkbox, it means an optout from the automatic topic subscriptions; the checkbox should not be checked by default next time they draft a reply in that section.)

The comment @Tacsipacsi posted above (T290712#7345502) and the conversations I've had with @iamjessklein (offline) and @Tribe of Tiger (at en.wiki) is helping to clarify for me the emerging tension between the Watch this page checkbox, peoples' Watchlists, and them subscribing to topics.

In this comment, I'm going to share the "clarity" I'm referring to, propose a path forward, and document the assumptions that are informing this path forward. I would value hearing what – if any – questions the below brings to mind for people...

Tension
The way Topic Subscriptions, and the Watch this page checkbox that appears within the Reply Tool and New Discussion Tool, is currently implemented is likely to lead people to wonder what exactly the Watch this page checkbox controls:

• "Does the checkbox control whether the page I am adding a comment or new discussion to will get added to my Watchlist?"
• "Does the checkbox control whether I will be subscribed to the conversation I am starting or commenting in ?"
• "Does the checkbox control whether I will be subscribed to the conversation I am starting or commenting in and whether said talk page will get added to my Watchlist?""

Path forward
In service of the "Assumptions" below, we will continue to ensure that people have the ability to Watch the talk page to which they are posting a comment to/starting a new discussion on and/or Subscribe to the topic they are starting/commenting within. More broadly, we will continue to think of, and design, page watching and topic subscriptions as "orthogonal" to one another, as @Tacsipacsi posted in T290712#7345502.

To ensure the above actions are clear to people, and to reconcile the "Tension" above, we will adjust the Watch this page language to read: Add page to your [Watchlist](INSERT SPECIAL:WATCHLIST URL HERE) in an effort to reduce the likelihood that people are uncertain the impact changing the setting will have.

Evaluating the "Path forward"
We will evaluate the effectiveness of the "Path forward", and potentially, discuss revisions to it as part of T290043.

Assumptions

1. People will appreciate adding a talk page to their Watchlist and Subscribing to a topic have distinct purposes.
2. In line with "1.", different people will want/need to wield these two tools/functions ("Watching" and "Subscribing") differently. At a minimum, we can assume people will need to be able to do one of the three following things after commenting in a discussion or starting a new one:
   • Senior Contributors who are accustomed to using/visiting their Watchlists to learn when there is new activity in a conversation they are interested in, will expect to be able to add the talk page on which the conversation they are participating is posted to their Watchilst.
   • Junior Contributors who are accustomed to using communication tools that have adopted more modern design patterns will expect to be able to elect to be notified when someone says something new in the conversation they are participating in (read: they will expect to be able to subscribe to the topic).
   • Senior Contributors who are interested in experimenting with being notified when a new comment is posted in a conversation they are participating in while still being able to depend on their Watchlist to learn when there is new activity on a [talk] page they are interested in, will expect to be able to add the talk page to their Watchlist and/or subscribe to the topic they are participating in.

Have you considered adding a second "Subscribe to this section"? I think that if you see:

• Add this page to your watchlist (i)
• Subscribe to this section (i)

side by side, you are likely to assume that these are different things.

Another option is to consider removing the watchlist item entirely. If we are going to consider that, then I think that the best time to consider that is after automatic topic subscriptions are available.

We could probably do some analysis on this after it's available -- see whether watchlisting of talk pages drops noticeably? If so that'd be a good sign that removing the watchlist item would be a good idea.

As discussed during today's team meeting, we're going to do the following...

Now

• Change the Watch this page language to Add this page to your [[Special:Watchlist|watchlist]] and keep the rest of the Advanced shelf as is. [i]

Future

• If/when we learn people do not find it intuitive what impact the "Add this page to your watchlist" checkbox has in relation to topic subscriptions, we will consider a minimum of the the following two interventions :
  • Adding an explicit affordance within the Advanced shelf for subscribing, as @Whatamidoing-WMF proposed in T290712#7355854
  • Removing the "Add this page to your watchlist" checkbox, provided the engagement data we are tracking [ii] confirms people are in fact NOT using it

...I've updated the task description to reflect the above.

i. Mockup showing the proposed change within the Reply Tool

ii. @DLynch confirmed we have logging in place to track how people are engaging with the Watch this page checkbox

Change 722566 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/extensions/DiscussionTools@master] Change the \"Watch this page\" label to distinguish it from topic subscriptions

https://gerrit.wikimedia.org/r/722566

Change 722566 merged by jenkins-bot:

[mediawiki/extensions/DiscussionTools@master] Change the \"Watch this page\" label to distinguish it from topic subscriptions

https://gerrit.wikimedia.org/r/722566