bugs - What is wrong with White Background for a DockedCell?

Bug introduced in V10.4 or earlier and persists through V11.3

---

SetOptions[
  EvaluationNotebook[]
, DockedCells -> Flatten @ {
    Cell[
      BoxData @ ToBoxes[Framed[InputForm@#, Background -> White]]
    , CellFrameMargins -> 15
    , CellMargins -> {{0, 0}, {0, 0}}
    , Background -> #

    ] & /@ {
      Inherited, Red, GrayLevel[1.], None, GrayLevel[.99], Blue
    }}
]

Apparently you can not have White Background for a DockedCells.

And if you do not obey they will take your margins, too!

Any reasonable explanation?

Notice the None case is documented:

If Background->None is set, then the margins are only applied if a CellFrame is present. If the cell frame is not drawn on all sides, then the margin will only be applied to the sides with a frame setting.

V11.2 Win7

[CASE:3975061] was created

Answer

[...] It does appear as though DockedCells cannot have White as their Background specification. I have filed a report with our developers regarding this incorrect limitation so that they may investigate and look to address the issue in a future release. [...]

- WRI Support

The workaround is to use 'almost' white:

SetOptions[EvaluationNotebook[]
, DockedCells -> Cell[
    BoxData @ ToBoxes[Framed[InputForm@GrayLevel[.999]]]
  , CellFrameMargins -> 15
  , CellMargins -> {{0, 0}, {0, 0}}
  , Background -> GrayLevel[.999]
  ]

]