fix(runtime-vapor): render slot fallback if provided content is invalid

packages/runtime-vapor/__tests__/componentSlots.spec.ts

@@ -502,5 +502,123 @@ describe('component: slots', () => {
       await nextTick()
       expect(host.innerHTML).toBe('<div><h1></h1><!--slot--></div>')
     })
+
+    test('render fallback when slot content is not valid', async () => {
+      const Child = {
+        setup() {
+          return createSlot('default', null, () =>
+            document.createTextNode('fallback'),
+          )
+        },
+      }
+
+      const { html } = define({
+        setup() {
+          return createComponent(Child, null, {
+            default: () => {
+              return template('<!--comment-->')()
+            },
+          })
+        },
+      }).render()
+
+      expect(html()).toBe('fallback<!--slot-->')
+    })
+
+    test('render fallback when v-if condition is false', async () => {
+      const Child = {
+        setup() {
+          return createSlot('default', null, () =>
+            document.createTextNode('fallback'),
+          )
+        },
+      }
+
+      const toggle = ref(false)
+
+      const { html } = define({
+        setup() {
+          return createComponent(Child, null, {
+            default: () => {
+              return createIf(
+                () => toggle.value,
+                () => {
+                  return document.createTextNode('content')
+                },
+              )
+            },
+          })
+        },
+      }).render()
+
+      expect(html()).toBe('fallback<!--if--><!--slot-->')
+
+      toggle.value = true
+      await nextTick()
+      expect(html()).toBe('content<!--if--><!--slot-->')
+
+      toggle.value = false
+      await nextTick()
+      expect(html()).toBe('fallback<!--if--><!--slot-->')
+    })
+
+    test('render fallback with nested v-if ', async () => {
+      const Child = {
+        setup() {
+          return createSlot('default', null, () =>
+            document.createTextNode('fallback'),
+          )
+        },
+      }
+
+      const outerShow = ref(false)
+      const innerShow = ref(false)
+
+      const { html } = define({
+        setup() {
+          return createComponent(Child, null, {
+            default: () => {
+              return createIf(
+                () => outerShow.value,
+                () => {
+                  return createIf(
+                    () => innerShow.value,
+                    () => {
+                      return document.createTextNode('content')
+                    },
+                  )
+                },
+              )
+            },
+          })
+        },
+      }).render()
+
+      expect(html()).toBe('fallback<!--if--><!--slot-->')
+
+      outerShow.value = true
+      await nextTick()
+      expect(html()).toBe('fallback<!--if--><!--if--><!--slot-->')
+
+      innerShow.value = true
+      await nextTick()
+      expect(html()).toBe('content<!--if--><!--if--><!--slot-->')
+
+      innerShow.value = false
+      await nextTick()
+      expect(html()).toBe('fallback<!--if--><!--if--><!--slot-->')
+
+      outerShow.value = false
+      await nextTick()
+      expect(html()).toBe('fallback<!--if--><!--slot-->')
+
+      outerShow.value = true
+      await nextTick()
+      expect(html()).toBe('fallback<!--if--><!--if--><!--slot-->')
+
+      innerShow.value = true
+      await nextTick()
+      expect(html()).toBe('content<!--if--><!--if--><!--slot-->')
+    })
   })
 })

packages/runtime-vapor/src/block.ts

@@ -67,16 +67,37 @@ export class DynamicFragment extends VaporFragment {
 
     if (this.fallback && !isValidBlock(this.nodes)) {
       parent && remove(this.nodes, parent)
-      this.nodes =
-        (this.scope || (this.scope = new EffectScope())).run(this.fallback) ||
-        []
+      // handle nested dynamic fragment
+      if (isFragment(this.nodes)) {
+        renderFallback(this.nodes, this.fallback, key)
+      } else {
+        this.nodes =
+          (this.scope || (this.scope = new EffectScope())).run(this.fallback) ||
+          []
+      }
       parent && insert(this.nodes, parent, this.anchor)
     }
 
     setActiveSub(prevSub)
   }
 }
 
+function renderFallback(
+  fragment: VaporFragment,
+  fallback: BlockFn,
+  key: any,
+): void {
+  if (fragment instanceof DynamicFragment) {
+    const nodes = fragment.nodes
+    if (isFragment(nodes)) {
+      renderFallback(nodes, fallback, key)
+    } else {
+      if (!fragment.fallback) fragment.fallback = fallback
+      fragment.update(fragment.fallback, key)
+    }
+  }
+}
+
 export function isFragment(val: NonNullable<unknown>): val is VaporFragment {
   return val instanceof VaporFragment
 }

packages/runtime-vapor/src/componentSlots.ts

@@ -126,6 +126,7 @@ export function createSlot(
     const renderSlot = () => {
       const slot = getSlot(rawSlots, isFunction(name) ? name() : name)
       if (slot) {
+        fragment.fallback = fallback
         // create and cache bound version of the slot to make it stable
         // so that we avoid unnecessary updates if it resolves to the same slot
         fragment.update(