(In reply to Mariusz Mazur from comment #1)
> Pre-DC codepaths did not have an issue like this at all until Michel Dänzer
> created this patch: https://patchwork.freedesktop.org/patch/209464/ for bug
> 105308 thereby introducing a problem with the same effects (DC monitor gets
> disconnected on wakeup, which on multi-display causes issues) via a quite
> different approach (a deliberate DRM_MODE_DPMS_OFF & ON). But that should be
> a separate bug, I think.

FWIW, that change should have no direct effect on whether the display is
considered connected or disconnected. The only change is that when the driver
is notified that a DP display is "disconnected" (which can also happen without
a physical disconnection, e.g. if the display is turned off), it doesn't
immediately turn off the GPU's DP source anymore, but waits until either
userspace asks to turn it off, or it gets notified that a display is
"connected" again. Both "disconnect" / "connect" hotplug events are sent to
userspace before and after this change. I suspect that immediately turning off
the DP source simply happened to delay sending the "disconnect" hotplug event
to userspace enough to avoid the issue on your system.