System Information
Operating system: Windows-10-10.0.17763-SP0 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.92

Blender Version
Broken: version: 2.90.0 Alpha, branch: master, commit date: 2020-07-04 15:31, hash: rB464aaf27016f
Worked: (newest version of Blender that worked as expected)

Short description of error

3d viewport in rendered mode (cycles) goes black every time a shader parameter is changed, but only if the 3d viewport is in it's own window on a second monitor

Exact steps for others to reproduce the error
duplicate 3d viewport to a new window, put it in rendered mode (cycles), change a material parameter. It updates interactively if the new window is on the same monitor as blender, but it goes black for a number of seconds during parameter editing if the window is on a second monitor.

see video