Debian stretch on a thinkpad x260, xfce w/ i3 window manager. Using a thinkpad ultra dock. Connecting to an external monitor through the HDMI works great, and the transition from two monitors to one, or from the external to the thinkpad screen, works great, too, until the computer is suspended. Once it enters suspend after having been connected to the monitor, it returns from suspend with a black screen. Only by reconnecting to the HDMI, using arandr, etc., can I get it back, but then I encounter the same problem again: it works only until it goes into suspend.
Any ideas? I tried updating the kernel. To no avail.