[MDX] Fix: GFM and Smartypants missing by default (#4588)

* fix: apply Astro defaults on empty md config

* chore: changeset

.changeset/weak-emus-confess.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/mdx': patch
+---
+
+Fix: Add GFM and Smartypants to MDX by default

packages/integrations/mdx/src/utils.ts

@@ -127,7 +127,7 @@ export async function getRemarkPlugins(
 		default:
 			remarkPlugins = [
 				...remarkPlugins,
-				...(config.markdown.extendDefaultPlugins ? DEFAULT_REMARK_PLUGINS : []),
+				...(markdownShouldExtendDefaultPlugins(config) ? DEFAULT_REMARK_PLUGINS : []),
 				...ignoreStringPlugins(config.markdown.remarkPlugins ?? []),
 			];
 			break;
@@ -162,7 +162,7 @@ export function getRehypePlugins(
 		default:
 			rehypePlugins = [
 				...rehypePlugins,
-				...(config.markdown.extendDefaultPlugins ? DEFAULT_REHYPE_PLUGINS : []),
+				...(markdownShouldExtendDefaultPlugins(config) ? DEFAULT_REHYPE_PLUGINS : []),
 				...ignoreStringPlugins(config.markdown.rehypePlugins ?? []),
 			];
 			break;
@@ -172,6 +172,13 @@ export function getRehypePlugins(
 	return rehypePlugins;
 }
 
+function markdownShouldExtendDefaultPlugins(config: AstroConfig): boolean {
+	return (
+		config.markdown.extendDefaultPlugins ||
+		(config.markdown.remarkPlugins.length === 0 && config.markdown.rehypePlugins.length === 0)
+	);
+}
+
 function ignoreStringPlugins(plugins: any[]) {
 	let validPlugins: PluggableList = [];
 	let hasInvalidPlugin = false;

packages/integrations/mdx/test/mdx-plugins.test.js

@@ -24,6 +24,17 @@ describe('MDX plugins', () => {
 		expect(selectTocLink(document)).to.not.be.null;
 	});
 
+	it('Applies GFM by default', async () => {
+		const fixture = await buildFixture({
+			integrations: [mdx()],
+		});
+
+		const html = await fixture.readFile(FILE);
+		const { document } = parseHTML(html);
+
+		expect(selectGfmLink(document)).to.not.be.null;
+	});
+
 	it('supports custom rehype plugins', async () => {
 		const fixture = await buildFixture({
 			integrations: [