fix(attr-fallthrough): ensure consistent attr fallthrough for root fragments with comments

fix #2549

packages/compiler-core/__tests__/transform.spec.ts

@@ -342,5 +342,24 @@ describe('compiler: transform', () => {
         )
       )
     })
+
+    test('multiple children w/ single root + comments', () => {
+      const ast = transformWithCodegen(`<!--foo--><div/><!--bar-->`)
+      expect(ast.codegenNode).toMatchObject(
+        createBlockMatcher(
+          FRAGMENT,
+          undefined,
+          [
+            { type: NodeTypes.COMMENT },
+            { type: NodeTypes.ELEMENT, tag: `div` },
+            { type: NodeTypes.COMMENT }
+          ] as any,
+          genFlagText([
+            PatchFlags.STABLE_FRAGMENT,
+            PatchFlags.DEV_ROOT_FRAGMENT
+          ])
+        )
+      )
+    })
   })
 })

packages/compiler-core/src/transform.ts

@@ -314,14 +314,23 @@ function createRootCodegen(root: RootNode, context: TransformContext) {
     }
   } else if (children.length > 1) {
     // root has multiple nodes - return a fragment block.
+    let patchFlag = PatchFlags.STABLE_FRAGMENT
+    let patchFlagText = PatchFlagNames[PatchFlags.STABLE_FRAGMENT]
+    // check if the fragment actually contains a single valid child with
+    // the rest being comments
+    if (
+      __DEV__ &&
+      children.filter(c => c.type !== NodeTypes.COMMENT).length === 1
+    ) {
+      patchFlag |= PatchFlags.DEV_ROOT_FRAGMENT
+      patchFlagText += `, ${PatchFlagNames[PatchFlags.DEV_ROOT_FRAGMENT]}`
+    }
     root.codegenNode = createVNodeCall(
       context,
       helper(FRAGMENT),
       undefined,
       root.children,
-      `${PatchFlags.STABLE_FRAGMENT} /* ${
-        PatchFlagNames[PatchFlags.STABLE_FRAGMENT]
-      } */`,
+      patchFlag + (__DEV__ ? ` /* ${patchFlagText} */` : ``),
       undefined,
       undefined,
       true