Nothing in particular causes this, it seems to just happen every now and again, even when I'm not doing anything (for example, it happened while I was writing the first draft of this issue, killing Chrome. I don't think I even moved my mouse...).

Each time, here are the errors from syslog:

JS WARNING: [/home/joe/.local/share/gnome-shell/extensions/paperwm@hedning:matrix.org/scratch.js 144]: reference to undefined property Symbol()

Followed immediately by:

JS WARNING: [/home/joe/.local/share/gnome-shell/extensions/paperwm@hedning:matrix.org/tiling.js 332]: reference to undefined property "_targetWidth"

Then gnome shell restarts. Same behaviour on both develop and master branches I think. I'm disabling the extension for now because this is quite frequent, but from what I've seen so far I love it!