**System Information**
Operating system: Windows 10 Pro (1909)
Graphics card: EVGA GTX 1050Ti 4GB
**Blender Version**
Broken: 3.0.0, 886196b88817, master, 2021-10-08 20:02 (and also 3.0.0, c73be23e1762, master 2021-06-17 00:06 although less noticeable due to the recent Cycles X improvements in the other version)
Worked: 2.93.0, 1a69d491e57c, master, 2021-05-24 15:29 (not the very latest from 2.93 but it's the one I have installed)
**Short description of error**
The viewport contents flash (very rapidly in Cycles X) when in the camera view and either a Render Region (Ctrl + B) is selected or Passepartout is at 1.000.
The flashing only occurs when the render/camera region is smaller than the 3D viewport size.
Tested both CPU and GPU rendering (CUDA mode), it happens in both.
**Full Video**
{F10912540}
**Shorter 60FPS video showing how fast it flashes**
(it actually hurts my eyes in person)
{F10912736}
**.blend file**
{F10912661}
**Exact steps for others to reproduce the error**
(for the attached .blend file skip to step 6)
1. create a new scene
2. create a camera if there isn't one
3. enter the camera view (numpad 0)
4. switch render engine to Cycles
5. enable Passepartout in the camera and set it to 1.000 or select a Render Region (Ctrl + B)
6. make sure you're in Rendered view mode
7. move the view around by shift-dragging with the middle mouse button
8. toggle Passepartout in the camera settings to see that the flashing stops