When you saw this, did it appear to delete the article in the review-changes diff view before saving? Or did it only happen after the save?

Yup, the deletions show in the preview screen:

Reported several times on frwiki too.

• https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:Le_Bistro/10_ao%C3%BBt_2025#Bug_de_l'%C3%A9diteur_visuel
• https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:Le_Bistro/25_juillet_2025#c-Lewisiscrazy-20250725084800-Bug_%C3%A9diteur?

As @Fugace said in the merged task, two users detailed that they encountered it for the first time on the 20 July (one of the problematic edits).
The previous deployment for frwiki (group2) appears to be on Thursday, July 17 were version went from 1.45.0-wmf.9 to 1.45.0-wmf.10. List of shipped changes: https://www.mediawiki.org/wiki/MediaWiki_1.45/wmf.10#VisualEditor + https://www.mediawiki.org/wiki/MediaWiki_1.45/wmf.10#Cite

@Esanders That does include all the converter refactoring, which is at least a bit suspicious.

I can't reproduce this with the 2010 or 2017 wikitext editor (I'm going as far as review changes).

Can anyone provide some reliable reproduction steps, maybe in a sandbox?

@Esanders : I think I might have an idea of why. I tried it right now with either Firefox or Edge, logged in or not, with the same results :

• Go to an article (for example, https://fr.wikipedia.org/wiki/Croix_de_Pelouse )
• Edit a section with wikicode (for example, I click on "modifier le code" near "Description")
• Edit something (add text, a source... Here I added a description https://fr.wikipedia.org/w/index.php?title=Croix_de_Pelouse&diff=prev&oldid=228113605 )
• Switch to visual editor *BY clicking "modifier" above the article* (if you switch with the little pen icon "changer d'éditeur", it seems it doesn't bug).

I hope this helps, I can't do better without screenshots.

Hi.
I had the issue before.
I can reproduce the problem on w:fr and w:en on condition that I am logged in.
If I am not logged in, the w:en editor behaves differently, I cannot switch to the editor after editing the wikicode without having to "leave the page" and thus lose the modifications.
Not sure if this is due to my preferences.
Thanks
Lew

Ahh, that does explain why I wasn't reproducing it. Further spillover from the switch to default arguments and their null/undefined behavior changes makes sense, I suppose.

Change #1184941 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/extensions/VisualEditor@master] DesktopArticleTarget: Fix switching editors with changes while editing a section

https://gerrit.wikimedia.org/r/1184941

I took a while writing that single-word patch because I tried to figure out in which places section can be null, undefined, or an empty string, and what exactly does this means in each place. Ultimately I gave up, it's complex and I don't want to spend a lot of time on it only to break some other corner case by accident. The patch fixes this particular scenario for me.

The whole mode switching business is extremely complex, as you can switch: with/without changes, between whole page/section edit/new section, with visual section editing enabled/disabled, from source mode/visual mode/old wikitext editor to source mode/visual mode/old wikitext editor, and I hope I'm not missing any combinations. Some of them are impossible, but it's hard to keep that all in mind. I remember that we broke switching one way or another so many times when I was working on VE more actively in the past.

I think this is one area that would really benefit from browser tests, even though they're unreliable.

Yes - this reminds me of the audit we did for mobile editing, which documents 2^4 = 16 different switching paths

tried to figure out in which places section can be null, undefined, or an empty string,

Once processed, section should always be string or null, as documented for example here:

/**
 * Switch to a different wikitext section
 *
 * @param {string|null} section Section to switch to: a number, 'T-'-prefixed number, 'new'
 *   or null (whole document)
 * @param {boolean} [noPrompt=false] Switch without prompting (changes will be lost either way)
 */
ve.init.mw.ArticleTarget.prototype.switchToWikitextSection = function ( section, noPrompt ) {

Change #1184941 merged by jenkins-bot:

[mediawiki/extensions/VisualEditor@master] DesktopArticleTarget: Fix switching editors with changes while editing a section

https://gerrit.wikimedia.org/r/1184941