Allow a BoxDecoration to be specified in ButtonThemeData

[jkurtw]

Internal: b/141787699

Flutter doesn't have a good way of decorating a button with something that isn't a flat color. This is because the underlying Material widget doesn't allow for it.

Right now we just stack a DecoratedBox behind a MaterialButton with a transparent background. This works for our larger buttons, but fails for our smaller buttons, since the MaterialButton pads the height up to 48dp. This makes the button look like a regular sized button and there's no reasonable way to workaround it.

A proper fix would be to allow a BoxDecoration to be specified in ButtonThemeData. Then we can remove the Stack + DecoratedBox workaround.

Related to or same as #21125

[jkurtw]

b/141787699

[shihaohong]

Hi @jkurtw, we shouldn't support this type of modification since that would violate the Material specification.

However, I think what you could be looking for is creating a custom button composed of a GestureDetector and a Container, similar to this S.O. post.

[lukepighetti]

@shihaohong Does it "violate" the Material spec or is it unspecified by Material spec?

[shihaohong]

I believe that modifying components to have an appearance not described in the design language is a violation of the design language.

[jkurtw]

/cc @willlarche for your thoughts

[github-actions]

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