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) ||

src/javascript/sourceMaps/SourceMapCodecProcess.ts

@@ -50,7 +50,11 @@ const execute = async (): Promise<string> => {
       throw cause;
     return JSON.stringify({
       state: "failure",
-      reason: cause instanceof SourceMapFormatFailure ? cause.reason : "format",
+      reason:
+        cause instanceof SourceMapFormatFailure &&
+        cause.reason !== "section-overlap"
+          ? cause.reason
+          : "format",
       message: cause.message,
     });
   }

src/javascript/sourceMaps/SourceMapFormat.ts

@@ -55,7 +55,7 @@ export interface SourceMapStructureOptions {
 /** A decoder-profile failure with an actionable producer constraint. */
 export class SourceMapFormatFailure extends Error {
   constructor(
-    readonly reason: "format" | "unsupported" | "limit",
+    readonly reason: "format" | "unsupported" | "limit" | "section-overlap",
     message: string,
   ) {
     super(message);

src/javascript/sourceMaps/TraceSourceMap.test.ts

@@ -310,7 +310,7 @@ describe("source-map layout admission", () => {
           { offset: { line: 0, column: 5 }, map: leaf },
         ],
       }),
-    ).toThrow(/overlap/u);
+    ).toThrow(expect.objectContaining({ reason: "section-overlap" }));
   });
   it("accepts absent optional names and null contents without changing reported identity", () => {
     expect(

src/javascript/sourceMaps/TraceSourceMap.ts

@@ -22,10 +22,7 @@ import {
   SourceMapFormatFailure,
   type SourceMapSourceDeclaration,
 } from "./SourceMapFormat.js";
-import {
-  decodeValidatedSourceMapLeaves,
-  isBeforeSourceMapLeafStop,
-} from "./DecodedSourceMap.js";
+import { decodeValidatedSourceMapLeaves } from "./DecodedSourceMap.js";
 
 /** Decode with the pinned upstream codec; preserve every equal-position candidate. */
 export const traceSourceMap = (
@@ -35,14 +32,6 @@ export const traceSourceMap = (
 ): WebSourceMapReport => {
   const inspected = inspectSourceMap(text);
   const decodedLeaves = decodeValidatedSourceMapLeaves(inspected.leaves, url);
-  for (const { leaf, rows } of decodedLeaves)
-    for (const [line, row] of rows.entries())
-      for (const segment of row)
-        if (!isBeforeSourceMapLeafStop(leaf, line, segment[0]))
-          throw new SourceMapFormatFailure(
-            "format",
-            "Embedded source-map mappings overlap the following section boundary.",
-          );
   let map: TraceMap;
   if (inspected.format === "regular") {
     const decoded = decodedLeaves[0];