Ubuntu 19.04 power management: deactivating screens on inactivity

Since upgrading to Ubuntu 19.04 (wm=i3, not gnome desktop), when my machine is idle for long enough the screens blank and neither the keyboard nor the mouse will awaken them. Up until that time, the host works normally.

I can still ssh into the machine, so it is not suspended. Indeed, via ssh I can see that what I do on the keyboard is still being accepted and acted upon by the host.

As an experiment, I put hooks in the pm-* utilities so that they would log their invocations and refuse to run (i.e., do nothing successfully). I also tried disabling xautolock to see if this is somehow related to i3-lock, but I still see the same behaviour.

If I quit (or kill) the i3 session, lightdm somehow wakes the screens.

I suspect this is not BIOS related, since pre-upgrade the host was behaving correctly.

So, in brief, this appears to be simply the screens being put into power-saving mode and not being wake-able.

FWIW, my graphics is this:

  *-display                 
       description: VGA compatible controller
       product: Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics Controller
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 06
       width: 64 bits
       clock: 33MHz
       capabilities: msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0
       resources: irq:34 memory:f7800000-f7bfffff memory:e0000000-efffffff ioport:f000(size=64) memory:c0000-dffff

Any suggestions for what to look at or how to debug this?