fix(ssr): fix ssr render output for fragment in slots

fix #5859

packages/server-renderer/__tests__/ssrSlot.spec.ts

@@ -0,0 +1,86 @@
+/**
+ * @jest-environment node
+ */
+
+import { createApp } from 'vue'
+import { renderToString } from '../src/renderToString'
+
+const components = {
+  one: {
+    template: `<div><slot/></div>`
+  }
+}
+
+describe('ssr: slot', () => {
+  test('text slot', async () => {
+    expect(
+      await renderToString(
+        createApp({
+          components,
+          template: `<one>hello</one>`
+        })
+      )
+    ).toBe(`<div><!--[-->hello<!--]--></div>`)
+  })
+
+  test('element slot', async () => {
+    expect(
+      await renderToString(
+        createApp({
+          components,
+          template: `<one><div>hi</div></one>`
+        })
+      )
+    ).toBe(`<div><!--[--><div>hi</div><!--]--></div>`)
+  })
+
+  test('empty slot', async () => {
+    expect(
+      await renderToString(
+        createApp({
+          components: {
+            one: {
+              template: `<div><slot/></div>`
+            }
+          },
+          template: `<one><template v-if="false"/></one>`
+        })
+      )
+    ).toBe(`<div><!--[--><!--]--></div>`)
+  })
+
+  test('multiple elements', async () => {
+    expect(
+      await renderToString(
+        createApp({
+          components,
+          template: `<one><div>one</div><div>two</div></one>`
+        })
+      )
+    ).toBe(`<div><!--[--><div>one</div><div>two</div><!--]--></div>`)
+  })
+
+  test('fragment slot (template v-if)', async () => {
+    expect(
+      await renderToString(
+        createApp({
+          components,
+          template: `<one><template v-if="true">hello</template></one>`
+        })
+      )
+    ).toBe(`<div><!--[--><!--[-->hello<!--]--><!--]--></div>`)
+  })
+
+  test('fragment slot (template v-if + multiple elements)', async () => {
+    expect(
+      await renderToString(
+        createApp({
+          components,
+          template: `<one><template v-if="true"><div>one</div><div>two</div></template></one>`
+        })
+      )
+    ).toBe(
+      `<div><!--[--><!--[--><div>one</div><div>two</div><!--]--><!--]--></div>`
+    )
+  })
+})

packages/server-renderer/src/helpers/ssrRenderSlot.ts

@@ -82,7 +82,7 @@ export function ssrRenderSlotInner(
   }
 }
 
-const commentRE = /^<!--.*-->$/
+const commentRE = /<!--.*?-->/g
 function isComment(item: SSRBufferItem) {
-  return typeof item === 'string' && commentRE.test(item)
+  return typeof item === 'string' && !item.replace(commentRE, '').trim()
 }