fix(matrix): ignore escaped backticks in mention masking

extensions/matrix/src/matrix/format.test.ts

@@ -219,6 +219,17 @@ describe("markdownToMatrixHtml", () => {
     expect(result.mentions).toEqual({});
   });
 
+  it("keeps escaped mentions literal after escaped backticks", async () => {
+    const result = await renderMarkdownToMatrixHtmlWithMentions({
+      markdown: "\\`literal then \\@alice:example.org",
+      client: createMentionClient(),
+    });
+
+    expect(result.html).toContain("`literal then @alice:example.org");
+    expect(result.html).not.toContain("matrix.to");
+    expect(result.mentions).toEqual({});
+  });
+
   it("restores escaped mentions in markdown link labels without linking them", async () => {
     const result = await renderMarkdownToMatrixHtmlWithMentions({
       markdown: "[\\@alice:example.org](https://example.com)",

extensions/matrix/src/matrix/format.ts

@@ -68,7 +68,7 @@ function maskEscapedMentions(markdown: string): string {
   let codeFenceLength = 0;
 
   while (idx < markdown.length) {
-    if (markdown[idx] === "`") {
+    if (markdown[idx] === "`" && !isMarkdownEscaped(markdown, idx)) {
       let runLength = 1;
       while (markdown[idx + runLength] === "`") {
         runLength += 1;
@@ -94,6 +94,16 @@ function maskEscapedMentions(markdown: string): string {
   return masked;
 }
 
+function isMarkdownEscaped(markdown: string, idx: number): boolean {
+  let slashCount = 0;
+  let cursor = idx - 1;
+  while (cursor >= 0 && markdown[cursor] === "\\") {
+    slashCount += 1;
+    cursor -= 1;
+  }
+  return slashCount % 2 === 1;
+}
+
 function restoreEscapedMentions(text: string): string {
   return text.replaceAll(ESCAPED_MENTION_SENTINEL, "@");
 }