Currently the panorama viewer supports the overlaid graphic used from 1.13-pre1 onwards. Before this, there was still such an overlay, implemented as a hardcoded overlay instead of an image file.
The version used from Beta 1.8 Pre-release to Beta 1.9 Prerelease 6 is described here: NeRdTheNed/MC-TextureGen#33
The version used from RC1 to 18w22c is described here: NeRdTheNed/MC-TextureGen#32
Both include the associated code for drawing the gradient.
Currently the panorama viewer supports the overlaid graphic used from 1.13-pre1 onwards. Before this, there was still such an overlay, implemented as a hardcoded overlay instead of an image file.
The version used from Beta 1.8 Pre-release to Beta 1.9 Prerelease 6 is described here: NeRdTheNed/MC-TextureGen#33
The version used from RC1 to 18w22c is described here: NeRdTheNed/MC-TextureGen#32
Both include the associated code for drawing the gradient.