fix(markdownlint-rules): preserve nested fenced blocks

Track complete opening fence markers and require closing markers to use the same character with at least the opening length.
Prevent shorter nested example fences from exposing code to prose checks and producing false one-sentence-per-line violations.
Add focused regression coverage for the shared prose iterator and cross-line sentence scan.

Author: cypher0n3

markdownlint-rules/utils.js

@@ -67,11 +67,11 @@ function* iterateNonFencedLines(lines) {
 
     const fenceMatch = trimmed.match(/^(```+|~~~+)/);
     if (fenceMatch) {
-      const marker = fenceMatch[1][0] === "`" ? "```" : "~~~";
+      const marker = fenceMatch[1];
       if (!inFence) {
         inFence = true;
         fenceMarker = marker;
-      } else if (fenceMarker === marker) {
+      } else if (fenceMarker[0] === marker[0] && marker.length >= fenceMarker.length) {
         inFence = false;
         fenceMarker = null;
       }
@@ -496,12 +496,12 @@ function* iterateLinesWithFenceInfo(lines) {
     const fenceMatch = trimmed.match(/^(```+|~~~+)/);
 
     if (fenceMatch) {
-      const marker = fenceMatch[1][0] === "`" ? "```" : "~~~";
+      const marker = fenceMatch[1];
       if (!inFence) {
         inFence = true;
         fenceMarker = marker;
         blockType = parseFenceInfo(line);
-      } else if (fenceMarker === marker) {
+      } else if (fenceMarker[0] === marker[0] && marker.length >= fenceMarker.length) {
         inFence = false;
         fenceMarker = null;
         blockType = "";

test/markdownlint-rules/one-sentence-per-line.test.js

@@ -182,6 +182,22 @@ describe("one-sentence-per-line", () => {
     assert.strictEqual(errors.length, 0);
   });
 
+  it("keeps shorter nested backtick fences inside the outer fenced block", () => {
+    const lines = [
+      "````markdown",
+      "```go",
+      "func estimateTokens(msgs []inference.Message) int {",
+      "    for _, message := range msgs {",
+      "    }",
+      "}",
+      "```",
+      "````",
+    ];
+    const config = { "one-sentence-per-line": { checkCrossLine: true } };
+    const errors = runRule(rule, lines, config);
+    assert.strictEqual(errors.length, 0);
+  });
+
   it("skips ATX headings", () => {
     const lines = ["## Heading with. Multiple parts."];
     const errors = runRule(rule, lines);

test/markdownlint-rules/utils.test.js

@@ -412,6 +412,24 @@ describe("utils", () => {
       assert.strictEqual(blocks[1][0].lineNumber, 6);
     });
 
+    it("does not close an outer backtick fence with a shorter nested fence", () => {
+      const lines = [
+        "Before fence.",
+        "````markdown",
+        "```go",
+        "Inside nested fence.",
+        "```",
+        "````",
+        "After fence.",
+      ];
+      const blocks = Array.from(iterateProseBlocks(lines));
+      assert.strictEqual(blocks.length, 2);
+      assert.deepStrictEqual(blocks.map((block) => block[0].line), [
+        "Before fence.",
+        "After fence.",
+      ]);
+    });
+
     it("preserves absolute line numbers", () => {
       const lines = [
         "",