puzzling.stackexchange.com tus left monitor black

This is a weird one.

This is a new issue on an existing Ubuntu 14.04, dual screen setup, using Unity, nothing out of the ordinary. When I go to puzzling.stackexchange.com on my left monitor screen in either chrome or firefox, that screen tus black.

This does not happen on any other website, but does happen on every page on that site (that I've tried). Going to that site in a browser on my right monitor screen doesn't have that effect. If I drag the browser from the right monitor onto the left one, the left monitor tus black once I've gotten it mostly over there. It starts showing stuff again when I drag it back.

My left screen (the one with the problem) is using an HDMI cable and the other screen is using a DVI cable. The problem persists across rebooting.

Puzzling SE recently got a new site design, documented here. What could be going on?

Running lshw -c video gives:

  *-display               
       description: VGA compatible controller
       product: Curacao XT [Radeon R7 370 / R9 270X/370 OEM]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:01:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:63 memory:e0000000-efffffff memory:f7e00000-f7e3ffff ioport:e000(size=256) memory:f7e40000-f7e5ffff

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