-- Lean 4 proof step from Mathlib.Geometry.RingedSpace.PresheafedSpace.HasColimits (goal)
Goal:
J : Type u'
inst : Category.{v', u'} J
C : Type u
inst_1 : Category.{v, u} C
inst_2 : HasColimitsOfShape.{v', u', v, v + 1} J TopCat
inst_3 : ∀ (X : TopCat.{v}), HasLimitsOfShape Jᵒᵖ (Presheaf C X)
inst_4 : HasLimitsOfShape.{v', u', v, u} Jᵒᵖ C
F : J ⥤ PresheafedSpace.{v, u, v} C
inst_5 : HasColimit F
U : Opens (↑(↑(Limits.colimit F) : TopCat) : Type v)
X Y : Jᵒᵖ
f : X ⟶ Y
⊢ Eq (α :=
    obj
        (Functor.comp (D := CategoryTheory.Functor.{v, v, v, u} (Opens (↑(Limits.colimit (F ⋙ forget C)) : Type v))ᵒᵖ C)
          (Functor.leftOp (pushforwardDiagramToColimit F) : Jᵒᵖ ⥤ Presheaf.{v, v, u} C (Limits.colimit (F ⋙ forget C)))
          (obj (evaluation (Opens (↑(Limits.colimit (F ⋙ forget C)) : Type v))ᵒᵖ C)
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U)) :
            CategoryTheory.Functor.{v, v, v, u} (Opens (↑(Limits.colimit (F ⋙ forget C)) : Type v))ᵒᵖ C ⥤ C))
        X ⟶
      (componentwiseDiagram F U).obj Y)
    (map
        (Functor.comp (D := CategoryTheory.Functor.{v, v, v, u} (Opens (↑(Limits.colimit (F ⋙ forget C)) : Type v))ᵒᵖ C)
          (Functor.leftOp (pushforwardDiagramToColimit F) : Jᵒᵖ ⥤ Presheaf.{v, v, u} C (Limits.colimit (F ⋙ forget C)))
          (obj (evaluation (Opens (↑(Limits.colimit (F ⋙ forget C)) : Type v))ᵒᵖ C)
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U)) :
            CategoryTheory.Functor.{v, v, v, u} (Opens (↑(Limits.colimit (F ⋙ forget C)) : Type v))ᵒᵖ C ⥤ C))
        f ≫
      Iso.hom (mapIso (F.obj (unop Y)).presheaf (eqToIso ⋯)))
    (Iso.hom (mapIso (F.obj (unop X)).presheaf (eqToIso ⋯)) ≫ (componentwiseDiagram F U).map f)
have naturality : Eq (α :=
  (F.obj (unop X)).presheaf.obj
      (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop X))))
        (obj
          (Functor.op
            (Opens.map
              (Hom.base
                (Iso.hom
                  (Iso.symm
                    (colimit.isoColimitCocone
                      { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
          (op U))) ⟶
    (F.obj (unop Y)).presheaf.obj (op (obj (Opens.map (Hom.base (colimit.ι F (unop Y)))) U)))
  (CategoryStruct.comp (Y :=
      (F.obj (unop Y)).presheaf.obj
        (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop Y))))
          (obj
            (Functor.op
              (Opens.map
                (Hom.base
                  (Iso.hom
                    (Iso.symm
                      (colimit.isoColimitCocone
                        { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
            (op U))))
      ((F.map f.unop).c.app
            (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop X))))
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U))) ≫
          NatTrans.app
              (Iso.inv
                (Pushforward.comp ((F ⋙ forget C).map f.unop) (colimit.ι (F ⋙ forget C) (unop X))
                  (F.obj (unop Y)).presheaf))
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U)) ≫
            NatTrans.app (Iso.hom (pushforwardEq sorry (F.obj (unop Y)).presheaf))
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U)) :
        (F.obj (unop X)).presheaf.obj
            (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop X))))
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U))) ⟶
          (F.obj (unop Y)).presheaf.obj
            (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop Y))))
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U))))
      ((F.obj (unop Y)).presheaf.map (Iso.hom (eqToIso sorry)) :
        (F.obj (unop Y)).presheaf.obj
            (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop Y))))
              (obj
                (Functor.op
                  (Opens.map
                    (Hom.base
                      (Iso.hom
                        (Iso.symm
                          (colimit.isoColimitCocone
                            { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
                (op U))) ⟶
          (F.obj (unop Y)).presheaf.obj (op (obj (Opens.map (Hom.base (colimit.ι F (unop Y)))) U))) :
    (F.obj (unop X)).presheaf.obj
        (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop X))))
          (obj
            (Functor.op
              (Opens.map
                (Hom.base
                  (Iso.hom
                    (Iso.symm
                      (colimit.isoColimitCocone
                        { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
            (op U))) ⟶
      (F.obj (unop Y)).presheaf.obj (op (obj (Opens.map (Hom.base (colimit.ι F (unop Y)))) U)))
  (Iso.hom (mapIso (F.obj (unop X)).presheaf (eqToIso sorry)) ≫ (componentwiseDiagram F U).map f :
    (F.obj (unop X)).presheaf.obj
        (obj (Functor.op (Opens.map (colimit.ι (F ⋙ forget C) (unop X))))
          (obj
            (Functor.op
              (Opens.map
                (Hom.base
                  (Iso.hom
                    (Iso.symm
                      (colimit.isoColimitCocone
                        { cocone := colimitCocone F, isColimit := colimitCoconeIsColimit F : ColimitCocone F }))))))
            (op U))) ⟶
      (F.obj (unop Y)).presheaf.obj (op (obj (Opens.map (Hom.base (colimit.ι F (unop Y)))) U)))
Tactic: change ((F.map f.unop).c.app _ ≫ _ ≫ _) ≫ (F.obj (unop Y)).presheaf.map _ = _ ≫ _