(I think a workaround would be not to use VE. Could not hold myself.)

(Also, I believe HotCat is no extension)

In T170896#3446750, @Base wrote:

(Also, I believe HotCat is no extension)

Is that another (misplaced) joke? Just checking, because of your previous comment.

In case it's not: in my preferences on nlwiki, it is at least listed under the extensions tab. Not sure what other definition you'd use.

Is there anything shown in the web browser's developer tools?
After opening them, please append ?debug=true to the address of the page on which you see the problem, if the address does not already include a ?. (If the address already includes a ?, append &debug=true instead - see https://www.mediawiki.org/wiki/ResourceLoader/Developing_with_ResourceLoader#Toggle_debug_mode for more information).
If there is a problem or an error with JavaScript it should be printed in the 'console' of the developer tools. Often JavaScript errors are a reason for problems, or a non-existing file called from another file (which can be seen in the 'network' tab).
For more information please see:

• https://en.wikipedia.org/wiki/Wikipedia:Reporting_JavaScript_errors
• https://developer.mozilla.org/en-US/docs/Tools/Web_Console for Firefox ≥24
• http://msdn.microsoft.com/en-us/library/ie/dn255006 for Internet Explorer
• https://developer.chrome.com/devtools for Chrome and Chromium
• https://developer.apple.com/library/mac/documentation/AppleApplications/Conceptual/Safari_Developer_Guide/Introduction/Introduction.html for Safari

At the moment I try to save the category, I get in debug mode the following (I get the same in regular mode btw) in my console:

VM3178:166 This page is using the deprecated ResourceLoader module "jquery.ui.position".
(anonymous) @ VM3178:166
(anonymous) @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:162
fire @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:45
add @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:45
always @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:46
runScript @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:162
checkCssHandles @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:162
execute @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:163
implement @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:169
(anonymous) @ VM3178:166
(anonymous) @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:4
globalEval @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:4
(anonymous) @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:167
VM3178:121 This page is using the deprecated ResourceLoader module "jquery.ui.core".
Please use "mediawiki.ui.button" or "oojs-ui" instead.
mw.loader.implement.css @ VM3178:121
(anonymous) @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:162
fire @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:45
add @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:45
always @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:46
runScript @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:162
checkCssHandles @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:162
(anonymous) @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:163
fire @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:45
fireWith @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:46
fire @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:46
fireCallbacks @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:157
addEmbeddedCSS @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:157
(anonymous) @ load.php?debug=false&lang=nl&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:157

This may be an issue in HotCat. I'm not sure.

I can reproduce this at the English Wikipedia.

The bug appears to be in HotCat - it uses wgCurRevisionId, but it makes a local copy of mw config: conf = mw.config.get() so even though VE updates wgCurRevisionId, HotCat doesn't see it.

Should be fixed by that gadget's community maintainers, whoever they are.

@TheDJ would that be you?

Fixed with https://commons.wikimedia.org/w/index.php?title=MediaWiki%3AGadget-HotCat.js&type=revision&diff=252283079&oldid=245282108

It looks to be User:Lupo (@Lupo). I guess follow-up should be on https://commons.wikimedia.org/wiki/MediaWiki_talk:Gadget-HotCat.js though I don't think it's very active.