I did an ordinary upgrade in Synaptic on each of a few Mepis 8.0 systems.

If I understand correctly, it replaced a bunch of KDM and KDE packages, some (maybe all) of which were Mepis specific, with the Debian base versions of the same packages. My main question is why did it do that?

One of the more visible results was a Debian login screen instead of a Mepis login screen and root GUI login prohibited.

I reenabled root GUI login and on one of the systems even changed the appearance of the login screen back to Mepis. But I'd like to know what is going on.

Is Mepis 8.0 obsolete enough that its differences from Debian are no longer supported? Earlier, I downloaded and tried a liveCD of Mepis 8.5 and generally didn't like the changes. I haven't had time to investigate further and see whether it can be tweaked back to something I like.

I was surprised to see nothing about this in forums. (On at least one of my Mepis 8.0 systems) I really don't think I have anything nonstandard in the repository selection or other config of Synaptic. So this ought to have happened to anyone who updated Mepis 8.0. Where are all the other complaints?

You should not be logging into X with a root account; it is becoming more common for modern desktop distributions to block this as there is absolutely no reason to do it. Ubuntu even disables the root account completely.

Though I am not sure why it would use a Debian-branded interface rather than Mepis.

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Could we not have that discussion here. I'm sure I can find that discussion in a few other threads at LQ.

I'd like to know why the update did the surprising thing it did.

You asked why it disables root logins on the GUI, and that is why. You aren't supposed to be doing that.

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I know that is not why Mepis 8.0 disabled root GUI login. I don't really care whether/why Debian disables root GUI login. I want to know why the ordinary upgrade process using Synaptic took away differences between Mepis 8.0 and Debian.

There are a couple threads about this in the mepislovers forum. If you accepted the "new" kdmrc file during updates, it overwrote the original mepis kdmrc file. Search for my post on kdmrc for a way to recover the original kdmrc file.
Cheers

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Apparently, I did a sloppy job of searching there before posting here. I should have found
http://mepislovers.org/showthread.php?t=26316

That update changed quite a lot of kde and kdm packages. The only change I understood and undid was to kdmrc. That also seems to be the only file for which undoing changes is being discussed at mepislovers.

So that means all the other changes are OK?

I'm not at any of my Mepis systems now to check your statement that seems to say the prior locally modified kdmrc would be available at /etc/kde3/kdm/kdmrc.dpkg-old
That should be the locally modified one (containing things like screen metrics adjustment), not the official Mepis one that was the starting point before local modification?

I don't actually need that this time (kdmrc is already back the way I want it on each system). But that sort of thing is nice to know for next time.

There another Thread now, too: http://mepislovers.org/forums/showthread.php?t=26394.

You're right! When I said the "original mepis kdmrc" I really just meant the one your system was using prior to the update.
Cheers

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My understanding is that the updates in question were Debian security related. The side effects were the Debian branded KDM amongst other things. In all likelyhood, one of the intentions was to disable GUI root logins for the (Valid!) security reasons that have been alluded to in this thread.