1. Open the reply or new topic widget.
2. Open the "advanced" tray
3. Observe the "watch this page" checkbox is not checked (this bug does not apply when the checkbox is automatically selected because of your preferences)
4. Use the watch star at the top of the page to watch the page
5. Observe the "watch this page" checkbox is still not checked
6. Post a comment
7. Observe the page has been unwatched

Expected:

5. The "watch this page" checkbox becomes checked

7. The page is remains watched after posting

Original report

List of steps to reproduce (step by step, including full links if applicable):

• Watch a user talk page (e.g. https://test.wikipedia.org/wiki/User_talk:TheresNoTime), either indefinitely or temporarily
• Click New Section, with the discussion tools enabled
• Fill in the section title, and message, and save

What happens?:
The message is posted, and the previously watched talk page is unwatched

What should have happened instead?:
The message is posted without affecting the watched status

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:

• Confirmed on both en.wikipedia and test.wikipedia
• This only appears to be affecting user talk pages

• Watching page API request

{
	"action": "watch",
	"format": "json",
	"formatversion": "2",
	"titles": "User_talk:TheresNoTime",
	"token": "x"
}

• Posting new talk page section request

-----------------------------
Content-Disposition: form-data; name="action"

discussiontoolsedit
-----------------------------
Content-Disposition: form-data; name="format"

json
-----------------------------
Content-Disposition: form-data; name="formatversion"

2
-----------------------------
Content-Disposition: form-data; name="uselang"

en
-----------------------------
Content-Disposition: form-data; name="paction"

addtopic
-----------------------------
Content-Disposition: form-data; name="errorformat"

html
-----------------------------
Content-Disposition: form-data; name="errorlang"

en
-----------------------------
Content-Disposition: form-data; name="errorsuselocal"

true
-----------------------------
Content-Disposition: form-data; name="page"

User_talk:TheresNoTime
-----------------------------
Content-Disposition: form-data; name="commentname"

null
-----------------------------
Content-Disposition: form-data; name="assert"

user
-----------------------------
Content-Disposition: form-data; name="assertuser"

TheresNoTime
-----------------------------
Content-Disposition: form-data; name="dtenable"

1
-----------------------------
Content-Disposition: form-data; name="dttags"

discussiontools,discussiontools-source,discussiontools-source-enhanced,discussiontools-newtopic
-----------------------------
Content-Disposition: form-data; name="wikitext"

test4
-----------------------------
Content-Disposition: form-data; name="watchlist"

unwatch
-----------------------------
Content-Disposition: form-data; name="sectiontitle"

test4
-----------------------------
Content-Disposition: form-data; name="token"

x
-------------------------------

of the above, the section

Content-Disposition: form-data; name="watchlist"

unwatch

appears to be of note. Contrasted to what happens when I do the same at, e.g. https://test.wikipedia.org/wiki/Template_talk:SPIpriorcases

Content-Disposition: form-data; name="watchlist"

watch

If it helps, my watchlist preferences on test.wikipedia are:

Advanced options

• Expand watchlist to show all changes, not just the most recent
• Add direct unwatch/watch markers (×/+) to watched pages with changes (JavaScript required for toggle functionality)
• Use non-JavaScript interface
• Loads Watchlist without filters search or highlighting functionality.
• Watch threads that I create or reply to
• Show Wikidata edits in your watchlist

Watched pages

• Add pages and files I edit to my watchlist
• Add pages and files I move to my watchlist
• Add pages and files I delete to my watchlist
• Add pages I create and files I upload to my watchlist
• Add pages where I have performed a rollback to my watchlist
• Add new files I upload to my watchlist