Should ExtensionBlock.fillna unbox Series / Index?

[TomAugspurger]

pandas/pandas/core/internals/blocks.py

Line 2056 in 9caf048

values = self.values if inplace else self.values.copy()

Should we get the ExtensionArray / ndarray backing the Series / Index at that point? As an EA author, I'd rather not have to worry about extracting the extension array from a Series in my fillna.

In the ExtensionArray.fillna docstring, we say the type can be 'arraylike'.

[jbrockmendel]

ATM we cannot ensure that Block.fillna will not get a Series. #32420 fixes that