test: assert corpus round-trip convergence for supported syntax

CONTRIBUTING.md

@@ -131,7 +131,7 @@ Pull request requirements:
 
 After submission, CI runs the automated checks. Maintainers apply a type label, assign the pull request's owner, and review the scope, implementation, and verification evidence; priority and Project status stay on the issue. Contributors should address review feedback or explain unresolved trade-offs. Maintainers squash merge accepted pull requests using the pull request title as the commit title on `main`. The pull request body becomes that commit's body and is the permanent record of the change; intermediate commits do not survive the merge. That is why the pull request body carries verification evidence and intermediate commit messages do not.
 
-Verify changes locally before merging. Use `pnpm check:frontend` for frontend-only work, `pnpm check:backend` for Rust/Tauri-only work, and `pnpm check` for cross-cutting updates. For manual testing of Markdown and the article navigator, open the committed `corpus/` directory or one of its focused scenario directories in the app.
+Verify changes locally before merging. Use `pnpm check:frontend` for frontend-only work, `pnpm check:backend` for Rust/Tauri-only work, and `pnpm check` for cross-cutting updates. For manual testing of Markdown and the article navigator, open the committed `corpus/` directory or one of its focused scenario directories in the app. Markdown round-trip convergence over `corpus/commonmark/`, `corpus/gfm/`, and `corpus/isolated/end-of-file/` is asserted automatically by `src/features/editor/tests/corpusRoundTrip.test.tsx`, so the manual pass over those directories covers rendering, interaction, and navigator behavior; the remaining subtrees are verified only by hand.
 
 Frontend checks enforce a coverage floor. It is a ratchet set just below the measured numbers rather than a target: a change that falls below it needs tests, not a lower floor, and the floor is raised when the measured numbers move up.
 

src/features/editor/tests/corpusRoundTrip.test.tsx

@@ -0,0 +1,51 @@
+import { readFileSync } from "node:fs";
+import { resolve } from "node:path";
+import { beforeEach, describe, expect, it } from "vitest";
+
+import { createMarkdownReferenceContext } from "@/test/factories/editor";
+import { setupMilkdownEditorMount } from "@/test/utils/milkdown";
+import { mockTauriApiCommand } from "@/test/utils/tauriApi";
+
+const mountEditor = setupMilkdownEditorMount(createMarkdownReferenceContext());
+
+const corpusFiles = [
+  "commonmark/blocks.md",
+  "commonmark/code.md",
+  "commonmark/emphasis.md",
+  "commonmark/html.md",
+  "commonmark/links-and-images.md",
+  "commonmark/lists-and-blockquotes.md",
+  "commonmark/text-and-breaks.md",
+  "gfm/autolinks.md",
+  "gfm/strikethrough.md",
+  "gfm/tables.md",
+  "gfm/tagfilter.md",
+  "gfm/task-lists.md",
+  "isolated/end-of-file/incomplete-html-comment.md",
+  "isolated/end-of-file/unclosed-code-fence.md",
+  "isolated/end-of-file/unclosed-directive.md",
+  "isolated/end-of-file/unclosed-html-block.md",
+];
+
+const readCorpusFile = (relativePath: string) =>
+  readFileSync(resolve(process.cwd(), "corpus", relativePath), "utf8");
+
+// The editor is allowed to normalize on first open, so the baseline is the first
+// serialization rather than the corpus file.
+describe("Corpus round trip", () => {
+  beforeEach(() => {
+    mockTauriApiCommand("resolveMarkdownImageTarget", ({ target }) => ({
+      kind: "renderable",
+      path: `C:/Notes/${target}`,
+    }));
+  });
+
+  it.each(corpusFiles)("converges on a stable serialization for %s", async (relativePath) => {
+    const source = readCorpusFile(relativePath);
+
+    const first = (await mountEditor(source)).getMarkdown();
+    const second = (await mountEditor(first)).getMarkdown();
+
+    expect(second).toBe(first);
+  });
+});