I am also seeing this, playing around with "modern" Gnome on a fresh install of Debian9/stretch amd64 (having previously been a user of the flashback mode up to Jessie).
Everything seems to work well, except if, from a logged in session, I lock the screen then hit the "pause" button in the top right controls on the lockscreen.... the screen goes blank and the system locks up. Any ssh sessions freeze too, so it's not just a display issue. Doesn't wake up again in response to any keyboard or mouse activity. Machine's power light stays on, but I have to do a long power-button press to get it to switch off so I can switch it on again. HW is a 7700K in a ASRock H270 Pro4 mobo. Not a laptop. Has 16GByte RAM and 16GByte swap. Not tried reading up on suspend issues generally yet. This machine is more of an always-on server, so I'm more concerned about naive users hitting that button accidentally than I am about getting it to work (ie for me a good solution would be being able to hide or disable it). Just posting this as a bit of supporting evidence the original report isn't a one off. Will follow up if/when I discover anything more.
