mouse crossing screens on dual monitors crashes X

[ExecutorElassus]

EDIT: I changed the title and edited the post, as I have since completely uninstalled Vivaldi, and the problem persists. So:

I can't seem to figure out what's causing the problem, but moving the mouse cursor across the screen boundary -- always from my right screen to the left one --- randomly causes X as a whole to crash out, dropping me back at the login screen and killing all the running applications that depend on a window.

In Xorg.0.log, I only see this:

[ExecutorElassus]

btw, it should be obvious from the log message, but I'm using nvidia-drivers-390.77. My WM is fvwm-2.6.5-r2, running as two separate instances.

Also, while we're at it, here's my /etc/X11/xorg.conf: