Responsive design and unexpected error messages

Thanks for the suggestion, but I do not think that is the answer. I have Update Channel set to 'Default', with 'Potentially incompatible extensions checkbox' set to 'Show'.
The inline help explains: 'Shows the checkbox in the pre–update check if any of the extensions installed on your site is potentially incompatible with the version of Joomla you are upgrading to. Note: the checkbox is displayed when upgrading to a new Joomla version family (minor or major version). '
As the pre-update check does not normally show when I do a Joomla! update I guessed an exisitng extension had some code that triggered it.  As Cedric has made a  local fix to my Slideshow extension I wondered if that might be the cause.

Hi
the pre-check process exists since Joomla 4.0
if you have the latest version of my extensions, you can pass through. The problem is that joomla will list all the plugins/modules but the info is only used in the component (for my extensions)
see here :
forum.joomlack.fr/index.php/3-boite-a-id...la-4-migration#61449

CEd

Hi CEd,
What puzzled me was that the pre-check screen was not shown when I did earlier Joomla updates, and 4.4.5 to 4.4.6 was the first time. I'm trying to track down the reason, as it is not obvious. You had made the fix to Slideshow CK under 4.4.5, so I wondered if that might have triggered it. I could be a problem with a completely different extension, but this seemed a good place to start. I'm being wary about forcing updates in case I break my site.

You had made the fix to Slideshow CK under 4.4.5, so I wondered if that might have triggered it

absolutely not
this is from Joomla core since 4.0, as said, and it's nothing related to any extension

That's fine. Something has changed since the update to 4.4.5, but tracking it down is not proving easy.
Alan