Hello,
This morning, I ran the latest Windows 10 (Enterprise, 64-bit) updates. After my computer rebooted, the whole Mattermost window is now black, except for the title bar (screenshot: http://imgur.com/tIz56mS).
I'm running version 3.4.1 and I also tried 3.5.0-RC2 and the problem is the same.
I don't know if this can help, but I've seen this problem once before: I was trying to enable triple monitor on my video card (NVIDIA NVS3100M) and the Optimus video drivers had all kinds of bugs (general Windows problems, and the Mattermost window was also black). I gave up and disabled Optimus in my BIOS and everything came back to normal, including Mattermost.