Fix indentation preservation in JSDoc (#37717)

src/compiler/parser.ts

@@ -6884,7 +6884,8 @@ namespace ts {
                     let state = JSDocState.SawAsterisk;
                     let margin: number | undefined;
                     // + 4 for leading '/** '
-                    let indent = start - Math.max(content.lastIndexOf("\n", start), 0) + 4;
+                    // + 1 because the last index of \n is always one index before the first character in the line and coincidentally, if there is no \n before start, it is -1, which is also one index before the first character
+                    let indent = start - (content.lastIndexOf("\n", start) + 1) + 4;
                     function pushComment(text: string) {
                         if (!margin) {
                             margin = indent;
@@ -6940,7 +6941,7 @@ namespace ts {
                                     comments.push(whitespace);
                                 }
                                 else if (margin !== undefined && indent + whitespace.length > margin) {
-                                    comments.push(whitespace.slice(margin - indent - 1));
+                                    comments.push(whitespace.slice(margin - indent));
                                 }
                                 indent += whitespace.length;
                                 break;

tests/cases/fourslash/jsDocIndentationPreservation1.ts

@@ -0,0 +1,13 @@
+///<reference path="fourslash.ts" />
+// @allowJs: true
+// @Filename: Foo.js
+
+/////**
+//// * Does some stuff.
+//// *     Second line.
+//// * 	Third line.
+//// */
+////function foo/**/(){}
+
+goTo.marker();
+verify.quickInfoIs("function foo(): void", "Does some stuff.\n    Second line.\n\tThird line.");

tests/cases/fourslash/jsDocIndentationPreservation2.ts

@@ -0,0 +1,13 @@
+///<reference path="fourslash.ts" />
+// @allowJs: true
+// @Filename: Foo.js
+
+/////**
+////	Does some stuff.
+////	    Second line.
+////		Third line.
+////*/
+////function foo/**/(){}
+
+goTo.marker();
+verify.quickInfoIs("function foo(): void", "Does some stuff.\n    Second line.\n\tThird line.");

tests/cases/fourslash/jsDocIndentationPreservation3.ts

@@ -0,0 +1,13 @@
+///<reference path="fourslash.ts" />
+// @allowJs: true
+// @Filename: Foo.js
+
+/////**
+////    Does some stuff.
+////        Second line.
+////    	Third line.
+////*/
+////function foo/**/(){}
+
+goTo.marker();
+verify.quickInfoIs("function foo(): void", "Does some stuff.\n    Second line.\n\tThird line.");