fix(runtime): preserve SSR default-slot content during hydration

src/runtime/client-hydrate.ts

@@ -697,12 +697,20 @@ const addSlottedNodes = (
 
   // Loop through the next siblings of the slot node, looking for nodes that match this slot's name
   do {
+    const slotAttribute = slottedNode?.['getAttribute'] ? slottedNode.getAttribute('slot') : null;
+    const isSlotFallback = slottedNode?.nodeName === 'SLOT-FB';
+    const ssrSlotName =
+      slottedNode && slottedNode.nodeType === NODE_TYPE.ElementNode
+        ? (slottedNode as Element).getAttribute('s-sn')
+        : null;
     if (
       slottedNode &&
-      (((slottedNode['getAttribute'] && slottedNode.getAttribute('slot')) || slottedNode['s-sn']) === slotName ||
+      !isSlotFallback &&
+      ((slotAttribute ?? slottedNode['s-sn']) === slotName ||
+        (slotAttribute === null && ssrSlotName === slotName) ||
         (slotName === '' &&
           !slottedNode['s-sn'] &&
-          (!slottedNode['getAttribute'] || !slottedNode.getAttribute('slot')) &&
+          slotAttribute === null &&
           (slottedNode.nodeType === NODE_TYPE.CommentNode || slottedNode.nodeType === NODE_TYPE.TextNode)))
     ) {
       // Looking for nodes that match this slot's name,

src/runtime/test/client-hydrate-to-vdom.spec.tsx

@@ -3,6 +3,7 @@ import { newSpecPage } from '@stencil/core/testing';
 
 import type * as d from '../../declarations';
 import { initializeClientHydrate } from '../client-hydrate';
+import { renderVdom } from '../vdom/vdom-render';
 
 describe('initializeClientHydrate', () => {
   it('functional', async () => {
@@ -83,4 +84,85 @@ describe('initializeClientHydrate', () => {
     expect(cmpAvnode.$children$).toHaveLength(1);
     expect(cmpAvnode.$children$[0].$text$.trim()).toBe('88mph');
   });
+
+  // Regression test for https://github.com/stenciljs/core/issues/6934.
+  it('collects SSR content in a nested non-shadow default slot before the owning host hydrates', async () => {
+    @Component({
+      tag: 'cmp-wrap',
+      shadow: false,
+    })
+    class CmpWrap {
+      render() {
+        return (
+          <Host>
+            <slot />
+          </Host>
+        );
+      }
+    }
+
+    @Component({
+      tag: 'cmp-page',
+      shadow: false,
+    })
+    class CmpPage {
+      render() {
+        return (
+          <cmp-wrap>
+            <div class="inner">Rendered by cmp-page</div>
+          </cmp-wrap>
+        );
+      }
+    }
+
+    const serverHydrated = await newSpecPage({
+      components: [CmpPage, CmpWrap],
+      html: `<cmp-page></cmp-page>`,
+      hydrateServerSide: true,
+    });
+    const container = document.createElement('div');
+    container.innerHTML = serverHydrated.root.outerHTML;
+
+    const wrapElm = container.querySelector('cmp-wrap') as d.HostElement;
+    const hostRef: d.HostRef = {
+      $flags$: 0,
+      $cmpMeta$: CmpWrap.COMPILER_META,
+      $hostElement$: wrapElm,
+    };
+    initializeClientHydrate(wrapElm, 'cmp-wrap', '2', hostRef);
+    renderVdom(hostRef, h('slot', null), true);
+
+    const inner = wrapElm.querySelector('.inner');
+    expect(inner?.hasAttribute('hidden')).toBe(false);
+  });
+
+  it('does not collect a serialized slot fallback from a foreign host', () => {
+    const hostElm = document.createElement('cmp-host') as d.HostElement;
+    hostElm.innerHTML = `
+      <!--r.1-->
+      <!--s.1.0.0.0.named-->
+      <slot-fb c-id="2.0.0.0" s-sn="named"></slot-fb>
+    `;
+    const foreignFallback = hostElm.querySelector('slot-fb') as d.RenderNode;
+    const hostRef: d.HostRef = { $flags$: 0 };
+
+    initializeClientHydrate(hostElm, 'cmp-host', '1', hostRef);
+
+    expect(foreignFallback['s-sn']).toBeUndefined();
+  });
+
+  it('does not let a serialized slot name override an explicit slot attribute', () => {
+    const hostElm = document.createElement('cmp-host') as d.HostElement;
+    hostElm.innerHTML = `
+      <!--r.1-->
+      <!--s.1.0.0.0.named-->
+      <div c-id="2.0.0.0" s-sn="named" slot="explicit"></div>
+    `;
+    const explicitlySlotted = hostElm.querySelector('div') as d.RenderNode;
+    const hostRef: d.HostRef = { $flags$: 0 };
+
+    initializeClientHydrate(hostElm, 'cmp-host', '1', hostRef);
+
+    expect(explicitlySlotted['s-sn']).toBeUndefined();
+  });
 });