fseditor.js crashes when PgDn key hit... sometimes

I hadn't seen one of these crashes in a while, but it happened just now:

The impossible happened: (line[l]) = {"text":"","attr":"","hardcr":false,"kludge
d":true,"firstchar":0,"selected":false}
line[l+1] = {"text":"","attr":"","hardcr":true,"kludged":false,"firstchar":0,"se
lected":false}
l 29