Fix an off-by-1 error in section lookup

lib/source-map-consumer.js

@@ -842,8 +842,11 @@ class IndexedSourceMapConsumer extends SourceMapConsumer {
           return cmp;
         }
 
+        // The generated column is 0-based, but the section offset column is
+        // stored 1-based.
         return (
-          aNeedle.generatedColumn - section.generatedOffset.generatedColumn
+          aNeedle.generatedColumn -
+          (section.generatedOffset.generatedColumn - 1)
         );
       }
     );

test/test-source-map-consumer.js

@@ -2183,3 +2183,12 @@ exports["test SourceMapConsumer.with and exceptions"] = async function (
   assert.equal(error, 6);
   assert.equal(consumer._mappingsPtr, 0);
 };
+
+exports["test a mapping at the boundary of indexed source map offset"] =
+  async function (assert) {
+    const map = await new SourceMapConsumer(
+      util.indexedTestMapAtOffsetBoundary
+    );
+    util.assertMapping(1, 0, "/the/root/one.js", 1, 0, null, null, map, assert);
+    map.destroy();
+  };

test/util.js

@@ -226,6 +226,31 @@ exports.indexedTestMapColumnOffset = {
     },
   ],
 };
+// This mapping is for testing a case where the mapped position is at the
+// section offset.
+exports.indexedTestMapAtOffsetBoundary = {
+  version: 3,
+  file: "min.js",
+  sections: [
+    {
+      offset: {
+        line: 0,
+        column: 0,
+      },
+      map: {
+        version: 3,
+        sources: ["one.js"],
+        sourcesContent: [
+          "ONE.foo = function (bar) {\n   return baz(bar);\n };",
+        ],
+        names: ["bar", "baz"],
+        mappings: "AAAA",
+        file: "min.js",
+        sourceRoot: "/the/root",
+      },
+    },
+  ],
+};
 exports.testMapWithSourcesContent = {
   version: 3,
   file: "min.js",