instance (priority := 100) {E : Type*} [NormedAddCommGroup E] [NormedSpace ℝ E]
    [FiniteDimensional ℝ E] : HasContDiffBump E := by
  refine' ⟨⟨_⟩⟩
  borelize E
  have IR : ∀ R : ℝ, 1 < R → 0 < (R - 1) / (R + 1) := by intro R hR; apply div_pos <;> linarith
  exact
    { toFun := fun R x => if 1 < R then y ((R - 1) / (R + 1)) (((R + 1) / 2)⁻¹ • x) else 0
      mem_Icc := fun R x => by
        simp only [mem_Icc]
        split_ifs with h
        · refine' ⟨y_nonneg _ _, y_le_one _ (IR R h)⟩
        · simp only [le_refl, zero_le_one, and_self]
      symmetric := fun R x => by
        simp only
        split_ifs
        · simp only [y_neg, smul_neg]
        · rfl
      smooth := by
        suffices
          ContDiffOn ℝ ⊤
            (uncurry y ∘ fun p : ℝ × E => ((p.1 - 1) / (p.1 + 1), ((p.1 + 1) / 2)⁻¹ • p.2))
            (Ioi 1 ×ˢ univ) by
          apply this.congr
          rintro ⟨R, x⟩ ⟨hR : 1 < R, _⟩
          simp only [hR, uncurry_apply_pair, if_true, Function.comp_apply]
        apply (y_smooth E).comp
        · apply ContDiffOn.prod
          · refine'
              (contDiffOn_fst.sub contDiffOn_const).div (contDiffOn_fst.add contDiffOn_const) _
            rintro ⟨R, x⟩ ⟨hR : 1 < R, _⟩
            apply ne_of_gt
            dsimp only
            linarith
          · apply ContDiffOn.smul _ contDiffOn_snd
            refine' ((contDiffOn_fst.add contDiffOn_const).div_const _).inv _
            rintro ⟨R, x⟩ ⟨hR : 1 < R, _⟩
            apply ne_of_gt
            dsimp only
            linarith
        · rintro ⟨R, x⟩ ⟨hR : 1 < R, _⟩
          have A : 0 < (R - 1) / (R + 1) := by apply div_pos <;> linarith
          have B : (R - 1) / (R + 1) < 1 := by apply (div_lt_one _).2 <;> linarith
          simp only [mem_preimage, prod_mk_mem_set_prod_eq, mem_Ioo, mem_univ, and_true_iff, A, B]
      eq_one := fun R hR x hx => by
        have A : 0 < R + 1 := by linarith
        simp only [hR, if_true]
        apply y_eq_one_of_mem_closedBall (IR R hR)
        simp only [norm_smul, inv_div, mem_closedBall_zero_iff, Real.norm_eq_abs, abs_div, abs_two,
          abs_of_nonneg A.le]
        calc
          2 / (R + 1) * ‖x‖ ≤ 2 / (R + 1) * 1 :=
            mul_le_mul_of_nonneg_left hx (div_nonneg zero_le_two A.le)
          _ = 1 - (R - 1) / (R + 1) := by field_simp; ring
      support := fun R hR => by
        have A : 0 < (R + 1) / 2 := by linarith
        have A' : 0 < R + 1 := by linarith
        have C : (R - 1) / (R + 1) < 1 := by apply (div_lt_one _).2 <;> linarith
        simp only [hR, if_true, support_comp_inv_smul₀ A.ne', y_support _ (IR R hR) C,
          _root_.smul_ball A.ne', Real.norm_of_nonneg A.le, smul_zero]
        refine' congr (congr_arg ball (Eq.refl 0)) _
        field_simp; 

Proof state:
E✝ : Type u_1
inst✝⁵ : NormedAddCommGroup E✝
inst✝⁴ : NormedSpace ℝ E✝
inst✝³ : FiniteDimensional ℝ E✝
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
this✝¹ : MeasurableSpace E := borel E
this✝ : BorelSpace E
IR : ∀ (R : ℝ), 1 < R → 0 < (R - 1) / (R + 1)
R : ℝ
hR : 1 < R
A : 0 < (R + 1) / 2
A' : 0 < R + 1
C : (R - 1) / (R + 1) < 1
⊢ (R + 1) * (R + R) = R * (2 * (R + 1))

Next tactic:
ring