This change could actually reduce the privacy of registered users. If "privileged user" means CheckUser, this will require massively increasing the number of CheckUsers needed to deal with routine day-to-day vandalism. A compromised (or rogue) CheckUser account is a disaster, so the privacy of registered users depends on keeping the number of such accounts as low as possible.

@Catrope: Now, in multiple browsers, document.body.scrollWidth > 10000 in MonoBook, producing a huge amount of empty space to the right of the content. Seemed to start with this fix.

@matmarex: Also tried in my sandbox, and it seems to work! And based on the latest data (created by piping variable dumps from an existing abuse filter to a throwaway Perl script), the problem seems to have stopped for everyone else.

Based on this extract from one of our section-blanking filters, it looks like this problem started on around 9 May, and is happening about 40 times per day on enwiki.

In T213763#4881554, @Bawolff wrote:

In principle, you should only get the session failure error if your token doesn't match your cookie, or if the request was missing some fields. In the event you have cookies disabled you should get a different error message. (I have not tested this myself - perhaps this code is broken and giving the wrong error message)

A very quick fix would be to restore the status quo, and temporarily remove the abusefilter-private right from all CheckUsers.

In T110329#4677511, @Deskana wrote:

In T110329#4676140, @Xaosflux wrote:

en wiki (https://en.wikipedia.org/w/index.php?title=Wikipedia:Edit_filter_noticeboard&oldid=864548831#VisualEditor) has created an edit filter to stop the rampant misuse of indexing in user space, that upon review we think is from this tool.

Do you have evidence of that, or is it a guess?
It would be relatively straightforward to figure that out, by seeing how many of the edits that add the magic word have the "Visual edit" tag on them.