I've just cross-posted this in the Slackware forum.
What I've learned from looking at the Xorg logs is that nvidia sets the mode to "Null" after kde starts. I am able to run xfce, and root is able to run KDE just fine (setting mode nvidia-auto-select). But for my main user, when running KDE, I get no signal right after the login.
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Edit: I ended up just removing my .kde folder entirely, then it did not crash on my next login.
(Well, I moved it to .kde-OLD, in case I wanted to use any of my previous settings.)
See "Sleep mode permanently turned video off" in the Slackware forum:
http://www.linuxquestions.org/questi...ff-4175609530/