I wonder why to even allow (=offer a UI element) to add the page to the watchlist while editing the very page......

To disallow page watchlisting in the middle of a page editing is also a possibility. One gets used to it. Perhaps the icon should be hidden in this case so the editors are not confused.

Anyway, the current state, with possible data loss, is unacceptable.

I cannot reproduce the issue. Clicking the star while editing adds/removes the page from the watchlist without interrupting my editing. The confirmation dialog should only show up when JS is disabled (which clearly isn't the case here) or updating the watched status in the background fails for some other reason (e.g. token expired). In this case indeed a new page is loaded, which will cause you lose all your editing. But in this case a warning should be shown before you leave the editing page (unless you disabled this warning in your preferences), and would happen just in the same way in the old wikitext editor.

@Schnark: an expired token (associated with the watchlist) is definitely a possibility as the trigger (see below). But anyway, NO warning is shown, the dialog is launched immediately (no meddling with the preferences).

Today, the dialog was launched for me when reading, for first unwatchlisting. Subsequent (un)watchlistings have not started the dialog. Btw. in this case I would expect returning to the original page (or at least an marked option for it) after (un)confirming (= end of the dialog). Not that I end on some "special" page where only marked option is to go to my watchlist.

I can't reproduce this either, although I did encounter another bug where the dialogue that's shown when you click edit is a bit too transparent.

For the clarity, this bug is not about usual popup window with watchlisting information (which is shown in the previous comment from @Deskana above). It's about dialog which is opened in the new page, and you are asked to confirm your (un)watchlisting choice. The trigger for the "unusual" dialog could be expired token (as hypothesised by Schnark). The problem is no warning if you are in middle of editing when you trigger the dialog.

In my previous comment, I made also another remark: after you go through the dialog, you should be redirected to the original page, or there should be at least a prominent option to go there (some big bold button) . Currently you end on a page where is only a confirmation message with an option to go to your watchlist.

In T173089#3541020, @Vachovec1 wrote:

For the clarity, this bug is not about usual popup window with watchlisting information (which is shown in the previous comment from @Deskana above). It's about dialog which is opened in the new page, and you are asked to confirm your (un)watchlisting choice.

Indeed. This was why I noted I could not reproduce this one, and that the bug I found was a different one. :-)