This has been merged with T259564.

There seems to be a race condition between the redirect to the newly edited page after the edit operation is completed and the callback to modify the watch state added as a DeferredUpdate during the edit. See here.

What's happening

1. When editing a page the callback to update the watchlist enters the queue at around position 94 and the redirect happens before the callback is executed
2. When editing a page if the edit is null (no changes to the article) but the watchlist state changes, the callback to update the watchlist enters the queue at around position 6 and everything works as expected

Below are some simplified logs of the order of execution of the different methods involved in editing the page, adding the callback to deferred updates, changing the watchlist state, and fetching the watched item. Look closely the request ID (3rd column) and timestamp at the end of the line. I've also added the deferred updates count right before we push WatchAction::doWatchOrUnwatch to the queue.

Editing a page and changing the watch status (current issue)
The third line is the new GET request to see the article after edit succeeds and runs before the callback is executed (4th line)

2020-08-12 22:02:47 [6045b840-be9c-4c0a-9242-f6292ee68d73] ... DEBUG: EditPage::edit: enter 1597269767.9306  
2020-08-12 22:02:48 [6045b840-be9c-4c0a-9242-f6292ee68d73] ... DEBUG: watchlistexpiry: deferred updates count: 93
2020-08-12 22:02:49 [bb508fca-9dc9-4f92-8fee-74b981f8871c] ... DEBUG: watchlistexpiry: WatchedItemStore::getWatchedItem enter 1597269769.4544 
2020-08-12 22:02:49 [6045b840-be9c-4c0a-9242-f6292ee68d73] ... DEBUG: watchlistexpiry: {closure} adding call to WatchAction::doWatchOrUnwatch 1597269769.5007
2020-08-12 22:02:49 [6045b840-be9c-4c0a-9242-f6292ee68d73] ... DEBUG: watchlistexpiry: WatchAction::doWatchOrUnwatch enter 1597269769.5015

Editing a page without changing the article and modifying the watch status (this works just fine)
Here everything runs in the expected order

2020-08-12 22:03:49 [a3a45848-5408-4897-a233-47f9693f72b0] ... DEBUG: EditPage::edit: enter 1597269829.7101  
2020-08-12 22:03:49 [a3a45848-5408-4897-a233-47f9693f72b0] ... DEBUG: watchlistexpiry: deferred updates count: 5  
2020-08-12 22:03:50 [a3a45848-5408-4897-a233-47f9693f72b0] ... DEBUG: watchlistexpiry: {closure} adding call to WatchAction::doWatchOrUnwatch 1597269830.0085  
2020-08-12 22:03:50 [a3a45848-5408-4897-a233-47f9693f72b0] ... DEBUG: watchlistexpiry: WatchAction::doWatchOrUnwatch enter 1597269830.0092  
2020-08-12 22:03:50 [29c1c491-aeed-421f-9b7c-e36b62e9c93b] ... DEBUG: watchlistexpiry: WatchedItemStore::getWatchedItem enter 1597269830.1748

Change 619899 had a related patch set uploaded (by Dmaza; owner: Dmaza):
[mediawiki/core@master] Fix race condition on edit page

https://gerrit.wikimedia.org/r/619899

Change 619899 merged by jenkins-bot:
[mediawiki/core@master] Fix race condition on edit page

https://gerrit.wikimedia.org/r/619899

Similar bug, but I think unrelated in terms of code T260434.

Tested all the possible (I think) transitions from one watch state to another while submitting an edit.

I did so for Source Editor (for an empty and non-empty edit) and VisualEditor.

After submitting, the watch star and tooltip were always in the correct state.

The only exception was T260434, but I think that is a different part of the code.

After each submit, I also refreshed the page to check that the correct watch state and expiry was actually saved to the database.

I also briefly tested with JavaScript disabled. It behaved the same, which is not surprising as this is not a JS-related issue.

Test environment: https://en.wikipedia.beta.wmflabs.org MediaWiki 1.36.0-alpha (473361f) 2020-08-17T06:40:15

N.B. I will make a note for myself to test this on production (probably testwiki) when it goes live.

Change 620980 had a related patch set uploaded (by Dmaza; owner: Dmaza):
[mediawiki/core@REL1_35] Fix race condition on edit page

https://gerrit.wikimedia.org/r/620980

Change 620980 merged by jenkins-bot:
[mediawiki/core@REL1_35] Fix race condition on edit page

https://gerrit.wikimedia.org/r/620980

In T259564#6388179, @dom_walden wrote:

N.B. I will make a note for myself to test this on production (probably testwiki) when it goes live.

I have retested this very briefly on testwiki (1.36.0-wmf.5 (rMW753f685df568) 04:37, 18 August 2020) on Source Editor and VisualEditor.