Bug 1662080

downgrade isn't possible on rawhide, rats.  So, I'm dead until this is fixed.

(In reply to Paul DeStefano from comment #1)
> downgrade isn't possible on rawhide, rats.  So, I'm dead until this is fixed.

Download the RPMs for systemd-* from a Fedora 29 site and 'rpm -Uvh --force --nodep' them onto your system, that will at least get you back up and running.  If you don't have an FTP or non-GUI browser (lynx or similar) client installed, you may need to download them to another machine and sneakernet them over on a USB stick.

ftp://mirror.cc.vt.edu/pub/fedora/linux/releases/29/Everything/x86_64/os/Packages/s/ or any other mirror should do it for you.

ncftp ...g/x86_64/os/Packages/s > pwd
  ftp://mirror.cc.vt.edu/pub/fedora/linux/releases/29/Everything/x86_64/os/Packages/s/
ncftp ...g/x86_64/os/Packages/s > dir systemd-*
-rw-r--r--   16 0        0      3868908   Jul 19 08:13   systemd-239-3.fc29.i686.rpm
-rw-r--r--   12 0        0      3763956   Jul 19 08:13   systemd-239-3.fc29.x86_64.rpm
-rw-r--r--    4 0        0        70508   Jul 15 21:41   systemd-bootchart-233-3.fc29.x86_64.rpm
-rw-r--r--   16 0        0       451396   Jul 19 08:12   systemd-container-239-3.fc29.i686.rpm
-rw-r--r--   12 0        0       436552   Jul 19 08:13   systemd-container-239-3.fc29.x86_64.rpm
-rw-r--r--    8 0        0       298140   Jul 19 08:13   systemd-devel-239-3.fc29.i686.rpm
-rw-r--r--    4 0        0       298148   Jul 19 08:13   systemd-devel-239-3.fc29.x86_64.rpm
-rw-r--r--    4 0        0        90688   Jul 19 08:13   systemd-journal-remote-239-3.fc29.x86_64.rpm
-rw-r--r--   16 0        0       517524   Jul 19 08:13   systemd-libs-239-3.fc29.i686.rpm
-rw-r--r--   12 0        0       488228   Jul 19 08:13   systemd-libs-239-3.fc29.x86_64.rpm
-rw-r--r--   12 0        0       167072   Jul 19 08:13   systemd-pam-239-3.fc29.x86_64.rpm
-rw-r--r--    4 0        0      4068848   Jul 19 08:13   systemd-tests-239-3.fc29.x86_64.rpm
-rw-r--r--   12 0        0      1320236   Jul 19 08:12   systemd-udev-239-3.fc29.x86_64.rpm

That should at least get you up and running.  Oh, and add 'exclude=systemd-*' to your /etc/dnf/dnf.conf so your next 'dnf update' doesn't re-wedge your box. Remember to remove it once a fixed systemd ships.

If you notice the problem within a couple of weeks you can go back using older composes. 
https://kojipkgs.fedoraproject.org/compose/rawhide/Fedora-Rawhide-20181222.n.0/compose/Everything/x86_64/os/ is still available and is what I used to downgrade.
https://kojipkgs.fedoraproject.org/compose/rawhide/ has the currently available composes, though not all went far enough to generate usable repos. Note that some didn't completely finish successfully, though still produced usable repos.

Another option is to go to koji builds for a package. Binaries from older builds are available for a while and you can download builds for a package using a web browser or koji (koji download-build --arch=x86_64 --arch=noarch systemd-239-10.git3bf819c.fc30).
Systemd is at:
https://koji.fedoraproject.org/koji/packageinfo?packageID=10477

Ah, cool, thanks.  That makes sense.

Now that I'm back at work, I see this doesn't seem to affect my work desktop. The home machine is a macbook air. The work machine is a dell workstation. It might be that more than one package was involved in the problem, so I'll retest the latest systemd again when I get home tonight.

I am still having the problem on my home machine. So an update of some other package didn't fix the problem and whatever the problem is, isn't affecting all systems.

Which desktop do you use?

I use XFCE on both the machine that works and the one that doesn't.
I use lxdm for the display manager on the machine that works.
I use lightdm on the one that doesn't.

Systemd 240-5 fixes this though I still have some artifacts in XFCE and I still have worse problems on my work machine which also uses XFCE (even when I switch to lightdm).