I believe Kevin is referring to the formatting of derivatives of tensors in the Tensorial Application. The following is a mockup of the formatting for tensors and the plain partial derivative of tensors. For tensors:
MakeBoxes[tensor[A_, up_List, down_List],
form : StandardForm | TraditionalForm] :=
Module[{newdown = down, newup = up, makespace},
makespace = If[#1 === Void, Style[#2, ShowContents -> False], #1] &;
newup = Inner[makespace, newup, newdown, List];
newdown =
Inner[makespace, newdown, newup,
List]; (InterpretationBox[#1, #2, Editable -> False,
BaseStyle -> {AutoMultiplicationSymbol -> False}
] &) @@ {SubsuperscriptBox[MakeBoxes[A, form],
RowBox[(MakeBoxes[#1, form] &) /@ newdown],
RowBox[(MakeBoxes[#1, form] &) /@ newup]], Tensor[A, up, down]}]
And for plain partial derivatives:
DifSym = ",";
MakeBoxes[partialD[t_, indices_],
form : StandardForm | TraditionalForm] :=
Module[{windx = Flatten[{indices}]},
If[MatchQ[t, tensor[_, _, _] | tensor[f_Symbol]],
InterpretationBox[#1, #2, Editable -> False] & @@ {SubscriptBox[
MakeBoxes[t, form],
RowBox[{DifSym, RowBox@(MakeBoxes[#, form] & /@ windx)}]],
partialD[t, indices]},
InterpretationBox[#1, #2, Editable -> False] & @@ {SubscriptBox[
RowBox[{"(", MakeBoxes[t, form], ")"}],
RowBox[{DifSym, RowBox@(MakeBoxes[#, form] & /@ windx)}]],
partialD[t, indices]}]]
The following statements format properly in Mathematica 9.0.1
tensor[A, {Void}, {a}]
partialD[tensor[A, {Void}, {a}], {j, k}]
I have found that the performance of Mathematica 10.1 is impaired enough that I have reverted to Mathematica 9.0.1.