Fix an off-by-one error in getBaseDirectoriesFromRootDirs

src/services/stringCompletions.ts

@@ -630,7 +630,7 @@ function getBaseDirectoriesFromRootDirs(rootDirs: string[], basePath: string, sc
 
     // Determine the path to the directory containing the script relative to the root directory it is contained within
     const relativeDirectory = firstDefined(rootDirs, rootDirectory =>
-        containsPath(rootDirectory, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectory.length) : undefined)!; // TODO: GH#18217
+        containsPath(rootDirectory, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectory.length + 1) : undefined)!; // TODO: GH#18217
 
     // Now find a path for each potential directory that is to be merged with the one containing the script
     return deduplicate<string>(

tests/cases/fourslash/completionForStringLiteralRelativeImport5.ts

@@ -0,0 +1,24 @@
+/// <reference path='fourslash.ts' />
+
+// Should give correct completions for directories when invoked from within nested folders
+
+// @rootDirs: /repo/src,/repo/generated
+
+// @Filename: /dir/secret_file.ts
+//// /*secret_file*/
+
+// @Filename: /repo/src/dir/test.ts
+//// import * as foo1 from ".//*import_as*/
+//// import foo2 = require(".//*import_equals*/
+//// var foo3 = require(".//*require*/
+
+// @Filename: /repo/generated/dir/f.ts
+//// /*f*/
+
+verify.completions(
+    {
+        marker: ["import_as", "import_equals", "require"],
+        exact: ["f"],
+        isNewIdentifierLocation: true,
+    }
+);