Change the behavior of clonable to be more opt-in

See the discussion here:

whatwg/html#10107 (comment)

The new consensus is that the old behavior was likely web-
incompatible, plus not very developer-desirable. The new behavior
adds a `shadowrootclonable` attribute for declarative shadow dom
to opt-in to clonable shadow roots.

This is a slight behavior change from the existing shipped
behavior, in that before the `clonable` concept was introduced,
*any* declarative shadow root within a `<template>` would be
cloned. Now, that behavior is opt in. So:

old:
  <template>
    <div>
      <template shadowrootmode=open>
        I do NOT get cloned!
      </template>
    </div>
  </template>

new:
  <template>
    <div>
      <template shadowrootmode=open shadowrootclonable>
        I get cloned!
      </template>
    </div>
  </template>

See these three spec PRs:
  whatwg/dom#1246
  whatwg/html#10069
  whatwg/html#10117

Bug: 1510466
Change-Id: Ice7c7579094eb08b882c4bb44f93045f23b8f222

resources/declarative-shadow-dom-polyfill.js

@@ -17,7 +17,8 @@ function polyfill_declarative_shadow_dom(root) {
   root.querySelectorAll("template[shadowrootmode]").forEach(template => {
     const mode = template.getAttribute("shadowrootmode");
     const delegatesFocus = template.hasAttribute("shadowrootdelegatesfocus");
-    const shadowRoot = template.parentNode.attachShadow({ mode, delegatesFocus });
+    const clonable = template.hasAttribute("shadowrootclonable");
+    const shadowRoot = template.parentNode.attachShadow({ mode, delegatesFocus, clonable });
     shadowRoot.appendChild(template.content);
     template.remove();
     polyfill_declarative_shadow_dom(shadowRoot);

shadow-dom/declarative/declarative-shadow-dom-basic.html

@@ -143,6 +143,29 @@
 }, 'Declarative Shadow DOM: delegates focus attribute');
 </script>
 
+test(() => {
+  const div = document.createElement('div');
+  div.setHTMLUnsafe(`
+    <div id="host">
+      <template shadowrootmode="open" shadowrootclonable>
+      </template>
+    </div>
+    `);
+  var host = div.querySelector('#host');
+  assert_true(!!host.shadowRoot,"No shadow root found");
+  assert_true(host.shadowRoot.clonable,"clonable should be true");
+  div.setHTMLUnsafe(`
+    <div id="host">
+      <template shadowrootmode="open">
+      </template>
+    </div>
+    `);
+  host = div.querySelector('#host');
+  assert_true(!!host.shadowRoot,"No shadow root found");
+  assert_false(host.shadowRoot.clonable,"clonable should be false without the shadowrootclonable attribute");
+}, 'Declarative Shadow DOM: clonable attribute');
+</script>
+
 <div id="multi-host" style="display:none">
   <template shadowrootmode="open">
     <span>root 1</span>

shadow-dom/declarative/declarative-shadow-dom-repeats.html

@@ -49,9 +49,9 @@
 </script>
 
 <div id=open2>
-  <template shadowrootmode=open shadowrootdelegatesfocus>
-    Open, delegates focus (not the default),
-    named slot assignment (the default), clonable (the default for declarative)
+  <template shadowrootmode=open shadowrootdelegatesfocus shadowrootclonable>
+    Open, delegates focus (not the default), clonable (not the default)
+    named slot assignment (the default)
   </template>
 </div>
 
@@ -66,11 +66,9 @@
   assert_throws_dom("NotSupportedError",() => {
     open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "manual", clonable: true});
   },'Mismatched shadow root slotAssignment should throw');
-  // See https://github.com/whatwg/html/issues/10107: the behavior of the
-  // clonable flag is still being discussed.
-  // assert_throws_dom("NotSupportedError",() => {
-  //   open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "named", clonable: false});
-  // },'Mismatched shadow root clonable should throw');
+  assert_throws_dom("NotSupportedError",() => {
+    open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "named", clonable: false});
+  },'Mismatched shadow root clonable should throw');
 
   const initialShadow = open2.shadowRoot;
   const shadow = open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "named", clonable: true}); // Shouldn't throw

shadow-dom/declarative/gethtml.tentative.html

@@ -9,7 +9,7 @@
 <body>
 
 <script>
-function testElementType(allowsShadowDom, elementType, runGetHTMLOnShadowRoot, declarativeShadowDom, mode, delegatesFocus, serializable) {
+function testElementType(allowsShadowDom, elementType, runGetHTMLOnShadowRoot, declarativeShadowDom, mode, delegatesFocus, serializable, clonable) {
   const t = test(t => {
     // Create and attach element
     let wrapper;
@@ -27,7 +27,7 @@
 
     let shadowRoot;
     const isOpen = mode === 'open';
-    let initDict = {mode: mode, delegatesFocus: delegatesFocus};
+    let initDict = {mode: mode, delegatesFocus: delegatesFocus, clonable};
     let expectedSerializable = null;
     switch (serializable) {
       case undefined: expectedSerializable = false; break;
@@ -37,9 +37,10 @@
     }
     const delegatesAttr = delegatesFocus ? ' shadowrootdelegatesfocus=""' : '';
     const serializableAttr = expectedSerializable ? ' serializable=""' : '';
+    const clonableAttr = clonable ? ' shadowrootclonable=""' : '';
 
     if (allowsShadowDom && declarativeShadowDom) {
-      const html = `<${elementType}><template shadowrootmode=${mode}${delegatesAttr}${serializableAttr}>`;
+      const html = `<${elementType}><template shadowrootmode=${mode}${delegatesAttr}${serializableAttr}${clonableAttr}>`;
       wrapper.setHTMLUnsafe(html);
       if (isOpen) {
         shadowRoot = wrapper.firstElementChild.shadowRoot;
@@ -58,11 +59,12 @@
     assert_true(!allowsShadowDom || !!shadowRoot);
 
     if (allowsShadowDom) {
-      const correctShadowHtml = `<template shadowrootmode="${mode}"${delegatesAttr}${serializableAttr}><slot></slot></template>`;
+      const correctShadowHtml = `<template shadowrootmode="${mode}"${delegatesAttr}${serializableAttr}${clonableAttr}><slot></slot></template>`;
       const correctHtml = `<${elementType}>${correctShadowHtml}</${elementType}>`;
       assert_equals(shadowRoot.mode,mode);
       assert_equals(shadowRoot.delegatesFocus,delegatesFocus);
       assert_equals(shadowRoot.serializable,expectedSerializable);
+      assert_equals(shadowRoot.clonable,clonable);
       shadowRoot.appendChild(document.createElement('slot'));
       const emptyElement = `<${elementType}></${elementType}>`;
       if (isOpen) {
@@ -91,7 +93,7 @@
     // ...and that the default for includeShadowRoots is false.
     assert_equals(wrapper.getHTML(),wrapper.innerHTML,'The default for includeShadowRoots should be false');
 
-  }, `${runGetHTMLOnShadowRoot ? 'ShadowRoot' : 'Element'}.getHTML() on <${elementType}>${allowsShadowDom ? `, with ${declarativeShadowDom ? 'declarative' : 'imperative'} shadow, mode=${mode}, delegatesFocus=${delegatesFocus}, serializable=${serializable}.` : ''}`);
+  }, `${runGetHTMLOnShadowRoot ? 'ShadowRoot' : 'Element'}.getHTML() on <${elementType}>${allowsShadowDom ? `, with ${declarativeShadowDom ? 'declarative' : 'imperative'} shadow, mode=${mode}, delegatesFocus=${delegatesFocus}, serializable=${serializable}, clonable=${clonable}.` : ''}`);
 }
 
 function runAllTests() {
@@ -103,9 +105,11 @@
       if (allowsShadowDom) {
         for (const declarativeShadowDom of [false, true]) {
           for (const delegatesFocus of [false, true]) {
-            for (const mode of ['open', 'closed']) {
-              for (const serializable of [undefined, 'false', 'true']) {
-                testElementType(true, elementName, runGetHTMLOnShadowRoot, declarativeShadowDom, mode, delegatesFocus, serializable);
+            for (const clonable of [false, true]) {
+              for (const mode of ['open', 'closed']) {
+                for (const serializable of [undefined, 'false', 'true']) {
+                  testElementType(true, elementName, runGetHTMLOnShadowRoot, declarativeShadowDom, mode, delegatesFocus, serializable, clonable);
+                }
               }
             }
           }

shadow-dom/shadow-root-clonable.html

@@ -47,14 +47,26 @@
   div.setHTMLUnsafe('<div><template shadowrootmode=open><input></template></div>');
   const root = div.firstElementChild.shadowRoot;
   assert_true(!!root);
-  assert_true(root.clonable, "clonable is automatically true for declarative shadow root");
+  assert_true(!root.clonable, "clonable is *not* automatically true for declarative shadow root");
+
+  const clone = div.cloneNode(true);
+  const clonedRoot = clone.firstElementChild.shadowRoot;
+  assert_true(!clonedRoot,'no shadow root gets cloned');
+}, "declarative shadow roots do *not* get clonable: true automatically");
+
+test(() => {
+  const div = document.createElement("div");
+  div.setHTMLUnsafe('<div><template shadowrootmode=open shadowrootclonable><input></template></div>');
+  const root = div.firstElementChild.shadowRoot;
+  assert_true(!!root);
+  assert_true(root.clonable, "clonable gets added when shadowrootclonable is present");
 
   const clone = div.cloneNode(true);
   const clonedRoot = clone.firstElementChild.shadowRoot;
   assert_true(!!clonedRoot);
   assert_equals(clonedRoot.children.length, 1, "children count");
   assert_equals(clonedRoot.children[0].localName, "input", "children content");
-}, "declarative shadow roots get clonable: true automatically");
+}, "declarative shadow roots can opt in to clonable with shadowrootclonable");
 </script>
 
 <template id="test">
@@ -70,8 +82,6 @@
   assert_true(!!root);
   const clone = template.content.cloneNode(true);
   const clonedRoot = clone.querySelector('#host').shadowRoot;
-  assert_true(!!clonedRoot);
-  assert_equals(clonedRoot.children.length, 1, "children count");
-  assert_equals(clonedRoot.children[0].localName, "input", "children content");
-}, "declarative shadow roots inside templates also get cloned automatically");
+  assert_true(!clonedRoot,'no shadow root gets cloned');
+}, "declarative shadow roots inside templates do *not* get cloned automatically");
 </script>