fix(runtime-core): properly diff slot node and fallback node

packages/runtime-core/__tests__/rendererOptimizedMode.spec.ts

@@ -8,6 +8,8 @@ import {
   createApp,
   createBlock,
   createCommentVNode,
+  createElementBlock,
+  createElementVNode,
   createTextVNode,
   createVNode,
   defineComponent,
@@ -697,6 +699,81 @@ describe('renderer: optimized mode', () => {
     )
   })
 
+  test('force full diff slot and fallback nodes', async () => {
+    const Comp = {
+      props: ['show'],
+      setup(props: any, { slots }: SetupContext) {
+        return () => {
+          return (
+            openBlock(),
+            createElementBlock('div', null, [
+              renderSlot(slots, 'default', { hide: !props.show }, () => [
+                true
+                  ? (openBlock(),
+                    (block = createElementBlock(
+                      Fragment,
+                      { key: 0 },
+                      [createTextVNode('foo')],
+                      PatchFlags.STABLE_FRAGMENT,
+                    )))
+                  : createCommentVNode('v-if', true),
+              ]),
+            ])
+          )
+        }
+      },
+    }
+
+    const show = ref(true)
+    const app = createApp({
+      render() {
+        return (
+          openBlock(),
+          createBlock(
+            Comp,
+            { show: show.value },
+            {
+              default: withCtx(({ hide }: { hide: boolean }) => [
+                !hide
+                  ? (openBlock(),
+                    createElementBlock(
+                      Fragment,
+                      { key: 0 },
+                      [
+                        createCommentVNode('comment'),
+                        createElementVNode(
+                          'div',
+                          null,
+                          'bar',
+                          PatchFlags.HOISTED,
+                        ),
+                      ],
+                      PatchFlags.STABLE_FRAGMENT,
+                    ))
+                  : createCommentVNode('v-if', true),
+              ]),
+              _: SlotFlags.STABLE,
+            },
+            PatchFlags.PROPS,
+            ['show'],
+          )
+        )
+      },
+    })
+
+    app.mount(root)
+    expect(inner(root)).toBe('<div><!--comment--><div>bar</div></div>')
+    expect(block).toBe(null)
+
+    show.value = false
+    await nextTick()
+    expect(inner(root)).toBe('<div>foo</div>')
+
+    show.value = true
+    await nextTick()
+    expect(inner(root)).toBe('<div><!--comment--><div>bar</div></div>')
+  })
+
   // 3569
   test('should force bailout when the user manually calls the slot function', async () => {
     const index = ref(0)

packages/runtime-core/src/helpers/renderSlot.ts

@@ -38,7 +38,12 @@ export function renderSlot(
       currentRenderingInstance!.parent.isCE)
   ) {
     if (name !== 'default') props.name = name
-    return createVNode('slot', props, fallback && fallback())
+    return createVNode(
+      'slot',
+      props,
+      fallback && fallback(),
+      fallback ? PatchFlags.BAIL : undefined,
+    )
   }
 
   let slot = slots[name]

packages/runtime-core/src/renderer.ts

@@ -829,7 +829,7 @@ function baseCreateRenderer(
       dynamicChildren = null
     }
 
-    if (dynamicChildren) {
+    if (optimized && dynamicChildren) {
       patchBlockChildren(
         n1.dynamicChildren!,
         dynamicChildren,
@@ -1062,11 +1062,14 @@ function baseCreateRenderer(
     let { patchFlag, dynamicChildren, slotScopeIds: fragmentSlotScopeIds } = n2
 
     if (
-      __DEV__ &&
-      // #5523 dev root fragment may inherit directives
-      (isHmrUpdating || patchFlag & PatchFlags.DEV_ROOT_FRAGMENT)
+      (__DEV__ &&
+        // #5523 dev root fragment may inherit directives
+        // HMR updated / Dev root fragment (w/ comments), force full diff
+        (isHmrUpdating ||
+          (patchFlag > 0 && patchFlag & PatchFlags.DEV_ROOT_FRAGMENT))) ||
+      // #9200 slot and fallback node has different patchFlag, force full diff
+      (n1 && n1.patchFlag !== n2.patchFlag)
     ) {
-      // HMR updated / Dev root fragment (w/ comments), force full diff
       patchFlag = 0
       optimized = false
       dynamicChildren = null
@@ -1101,6 +1104,7 @@ function baseCreateRenderer(
       )
     } else {
       if (
+        optimized &&
         patchFlag > 0 &&
         patchFlag & PatchFlags.STABLE_FRAGMENT &&
         dynamicChildren &&