Fix Markdown task list indentation drift (#19647)

# Conflicts:
#	tests/format/markdown/list/__snapshots__/format.test.js.snap

tests/format/markdown/list/__snapshots__/format.test.js.snap

@@ -1480,7 +1480,7 @@ tabWidth: 4
 
 - foo
 
-      indented code block in bullet
+        indented code block in bullet
 
 ---
 
@@ -1516,7 +1516,7 @@ tabWidth: 4
 
 - [ ] foo
 
-      indented code block in checkbox item
+        indented code block in checkbox item
 
 ---
 
@@ -1758,7 +1758,7 @@ tabWidth: 999
 
 - foo
 
-      indented code block in bullet
+         indented code block in bullet
 
 ---
 
@@ -1782,7 +1782,7 @@ tabWidth: 999
 
 1. foo
 
-       indented code block in numbered item
+          indented code block in numbered item
 
 ---
 
@@ -1794,7 +1794,7 @@ tabWidth: 999
 
 - [ ] foo
 
-      indented code block in checkbox item
+         indented code block in checkbox item
 
 ---
 
@@ -1861,7 +1861,7 @@ tabWidth: 999
         -    item 1-1
         -    item 1-2
 
-         indented code block
+            indented code block
 
     top level indented code block
 
@@ -4092,6 +4092,89 @@ proseWrap: "always"
 ================================================================================
 `;
 
+exports[`issue-19644.md - {"proseWrap":"always","tabWidth":0} format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+proseWrap: "always"
+tabWidth: 0
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+- [x] short first line.
+
+      second paragraph at six spaces that wraps
+      onto another line here.
+
+=====================================output=====================================
+- [x] short first line.
+
+      second paragraph at six spaces that wraps
+      onto another line here.
+
+================================================================================
+`;
+
+exports[`issue-19644.md - {"proseWrap":"always","tabWidth":4} format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+proseWrap: "always"
+tabWidth: 4
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+- [x] short first line.
+
+      second paragraph at six spaces that wraps
+      onto another line here.
+
+=====================================output=====================================
+- [x] short first line.
+
+        second paragraph at six spaces that wraps
+        onto another line here.
+
+================================================================================
+`;
+
+exports[`issue-19644.md - {"proseWrap":"always","tabWidth":999} format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+proseWrap: "always"
+tabWidth: 999
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+- [x] short first line.
+
+      second paragraph at six spaces that wraps
+      onto another line here.
+
+=====================================output=====================================
+- [x] short first line.
+
+         second paragraph at six spaces that wraps
+         onto another line here.
+
+================================================================================
+`;
+
+exports[`issue-19644.md - {"proseWrap":"always"} format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+proseWrap: "always"
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+- [x] short first line.
+
+      second paragraph at six spaces that wraps
+      onto another line here.
+
+=====================================output=====================================
+- [x] short first line.
+
+      second paragraph at six spaces that wraps
+      onto another line here.
+
+================================================================================
+`;
+
 exports[`long-paragraph.md - {"proseWrap":"always","tabWidth":0} format 1`] = `
 ====================================options=====================================
 parsers: ["markdown"]

tests/format/markdown/list/issue-19644.md

@@ -0,0 +1,4 @@
+- [x] short first line.
+
+      second paragraph at six spaces that wraps
+      onto another line here.