/-- The derivative of the zero section of the tangent bundle maps `⟨x, v⟩` to `⟨⟨x, 0⟩, ⟨v, 0⟩⟩`.

Note that, as currently framed, this is a statement in coordinates, thus reliant on the choice
of the coordinate system we use on the tangent bundle.

However, the result itself is coordinate-dependent only to the extent that the coordinates
determine a splitting of the tangent bundle.  Moreover, there is a canonical splitting at each
point of the zero section (since there is a canonical horizontal space there, the tangent space
to the zero section, in addition to the canonical vertical space which is the kernel of the
derivative of the projection), and this canonical splitting is also the one that comes from the
coordinates on the tangent bundle in our definitions. So this statement is not as crazy as it
may seem.

TODO define splittings of vector bundles; state this result invariantly. -/
theorem tangentMap_tangentBundle_pure (p : TangentBundle I M) :
    tangentMap I I.tangent (zeroSection E (TangentSpace I)) p = ⟨⟨p.proj, 0⟩, ⟨p.2, 0⟩⟩ := by
  rcases p with ⟨x, v⟩
  have N : I.symm ⁻¹' (chartAt H x).target ∈ 𝓝 (I ((chartAt H x) x)) := by
    apply IsOpen.mem_nhds
    apply (PartialHomeomorph.open_target _).preimage I.continuous_invFun
    simp only [mfld_simps]
  have A : MDifferentiableAt I I.tangent (fun x => @TotalSpace.mk M E (TangentSpace I) x 0) x :=
    haveI : Smooth I (I.prod 𝓘(𝕜, E)) (zeroSection E (TangentSpace I : M → Type _)) :=
      Bundle.smooth_zeroSection 𝕜 (TangentSpace I : M → Type _)
    this.mdifferentiableAt
  have B :
    fderivWithin 𝕜 (fun x' : E => (x', (0 : E))) (Set.range I) (I ((chartAt H x) x)) v = (v, 0)
  · rw [fderivWithin_eq_fderiv, DifferentiableAt.fderiv_prod]
    · simp
    · exact differentiableAt_id'
    · exact differentiableAt_const _
    · exact ModelWithCorners.unique_diff_at_image I
    · exact differentiableAt_id'.prod (differentiableAt_const _)
  simp (config := { unfoldPartialApp := true }) only [Bundle.zeroSection, tangentMap, mfderiv, A,
    if_pos, chartAt, FiberBundle.chartedSpace_chartAt, TangentBundle.trivializationAt_apply,
    tangentBundleCore, Function.comp_def, ContinuousLinearMap.map_zero, mfld_simps]
  rw [← fderivWithin_inter N] at B
  rw [← fderivWithin_inter N, ← B]
  congr 1
  refine' fderivWithin_congr (fun y hy => _) _
  · simp only [mfld_simps] at hy
    

Proof state:
case mk.e_a.refine'_1
𝕜 : Type u_1
inst✝²⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝²³ : NormedAddCommGroup E
inst✝²² : NormedSpace 𝕜 E
H : Type u_3
inst✝²¹ : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝²⁰ : TopologicalSpace M
inst✝¹⁹ : ChartedSpace H M
Is : SmoothManifoldWithCorners I M
E' : Type u_5
inst✝¹⁸ : NormedAddCommGroup E'
inst✝¹⁷ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝¹⁶ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝¹⁵ : TopologicalSpace M'
inst✝¹⁴ : ChartedSpace H' M'
I's : SmoothManifoldWithCorners I' M'
F : Type u_8
inst✝¹³ : NormedAddCommGroup F
inst✝¹² : NormedSpace 𝕜 F
G : Type u_9
inst✝¹¹ : TopologicalSpace G
J : ModelWithCorners 𝕜 F G
N✝ : Type u_10
inst✝¹⁰ : TopologicalSpace N✝
inst✝⁹ : ChartedSpace G N✝
Js : SmoothManifoldWithCorners J N✝
F' : Type u_11
inst✝⁸ : NormedAddCommGroup F'
inst✝⁷ : NormedSpace 𝕜 F'
G' : Type u_12
inst✝⁶ : TopologicalSpace G'
J' : ModelWithCorners 𝕜 F' G'
N' : Type u_13
inst✝⁵ : TopologicalSpace N'
inst✝⁴ : ChartedSpace G' N'
J's : SmoothManifoldWithCorners J' N'
F₁ : Type u_14
inst✝³ : NormedAddCommGroup F₁
inst✝² : NormedSpace 𝕜 F₁
F₂ : Type u_15
inst✝¹ : NormedAddCommGroup F₂
inst✝ : NormedSpace 𝕜 F₂
f f₁ : M → M'
s s₁ t : Set M
x✝ : M
m n : ℕ∞
x : M
v : TangentSpace I x
N : ↑(ModelWithCorners.symm I) ⁻¹' (chartAt H x).toPartialEquiv.target ∈ 𝓝 (↑I (↑(chartAt H x) x))
A : MDifferentiableAt I (ModelWithCorners.tangent I) (fun x => { proj := x, snd := 0 }) x
B :
  (fderivWithin 𝕜 (fun x' => (x', 0)) (range ↑I ∩ ↑(ModelWithCorners.symm I) ⁻¹' (chartAt H x).toPartialEquiv.target)
        (↑I (↑(chartAt H x) x)))
      v =
    (v, 0)
y : E
hy : y ∈ range ↑I ∧ ↑(ModelWithCorners.symm I) y ∈ (chartAt H x).toPartialEquiv.target
⊢ (↑I (↑(ChartedSpace.chartAt x) (↑(PartialHomeomorph.symm (ChartedSpace.chartAt x)) (↑(ModelWithCorners.symm I) y))),
      0) =
    (y, 0)

Next tactic:
simp only [hy, Prod.mk.inj_iff, mfld_simps]