fix(browser): reject overlapping indexed source maps

Fixes #6562

src/javascript/sourceMaps/DecodedSourceMap.ts

@@ -5,7 +5,7 @@ import {
 } from "./SourceMapFormat.js";
 
 /** Determine whether a leaf-local mapping falls before its enclosing boundary. */
-export const isBeforeSourceMapLeafStop = (
+const isBeforeSourceMapLeafStop = (
   leaf: SourceMapLeaf,
   line: number,
   column: number,
@@ -40,6 +40,11 @@ export const decodeValidatedSourceMapLeaves = (
         const generatedLine = leaf.offset.line + line;
         const generatedColumn =
           segment[0] + (line === 0 ? leaf.offset.column : 0);
+        if (!isBeforeSourceMapLeafStop(leaf, line, segment[0]))
+          throw new SourceMapFormatFailure(
+            "section-overlap",
+            "Embedded source-map mappings overlap the following section boundary.",
+          );
         if (
           !Number.isSafeInteger(generatedLine) ||
           !Number.isSafeInteger(generatedColumn) ||