Today I had to change the layer named “UIToolkit” from “User Layer 8” in Edit Layers (Inspector) to “User Layer 10” to avoid conflicts with multiple objects in a coworker’s .unitypackages that use “User Layer 8” under a different Layer name.
As soon as I did this, weird things start showing up in the Game panel when I hit play. A rectangular flash started appearing in the bottom right corner as the main UIToolkit background image Sprite faded in at Scene launch.
The culprit was the Main Camera’s Culling Mask. As soon as I removed the UIToolkit layer from the Culling Mask in the Inspector the Main Camera stopped rendering a piece of the background image Sprite and this rectangle flash issue went away.