Stax, 7 bytes

ö÷e╧╤ù◘

Run and debug it

Explanation

|]miTiNt
|]m      map each suffix to:
   iT    remove iteration count elements from right
     iNt pad with iteration count number of spaces on left

Stax, 7 bytes

ö÷e╧╤ù◘

Run and debug it

Explanation

|]miTiNt
|]m      map each suffix to:
   iT    remove iteration count elements from right
     iNt pad with iteration count number of spaces on left

Stax, 117 bytes

c%{d
   xitiT|c
   {|,}20*|:xi{xitiTx%|CPFxP
   {|,}20*|:xi{xitiTx%|CPFxitiNtP
   {|,}20*|:xi{xitiTx%|CPFxitiTx%|CP
F

Run and debug it, animated

The more fun version. Since stax supports requestAnimationFrame from js, |, and |: can be used to animate each step.

Stax, 7 bytes

ö÷e╧╤ù◘

Run and debug it

Explanation

|]miTiNt
|]m      map each suffix to:
   iT    remove iteration count elements from right
     iNt pad with iteration count number of spaces of left

Stax, 7 bytes

ö÷e╧╤ù◘

Run and debug it

Explanation

|]miTiNt
|]m      map each suffix to:
   iT    remove iteration count elements from right
     iNt pad with iteration count number of spaces on left