Scope ModelContext to Document, and handle detached documents

index.bs

@@ -86,6 +86,15 @@ spec:html; type:dfn;
   text:browsing context group set
   text:unique internal value
 </pre>
+<pre class="anchors">
+spec: html; type: dfn; urlPrefix: https://html.spec.whatwg.org/C
+  text: create and initialize a Document object; url: initialise-the-document-object
+  # Because `text` and `url` are the same (modulo slashes), we should be able to
+  # exclude `url`. But the colon breaks the URL in `text`, forcing us to include
+  # `url`.
+  text:is initial about:blank; url: is-initial-about:blank
+  text: associated Navigator
+</pre>
 
 <h2 id="intro">Introduction</h2>
 
@@ -113,6 +122,7 @@ A <dfn>model context</dfn> is a [=struct=] with the following [=struct/items=]:
      [=structs=].
 </dl>
 
+
 A <dfn>tool definition</dfn> is a [=struct=] with the following [=struct/items=]:
 
 <dl dfn-for="tool definition">
@@ -183,19 +193,47 @@ https://dlaliberte.github.io/bikeshed-intro/#a-strategy-for-incremental-developm
 
 <h3 id="navigator-extension">Extensions to the {{Navigator}} Interface</h3>
 
-The {{Navigator}} interface is extended to provide access to the {{ModelContext}}.
+Each {{Navigator}} object has an <dfn for=Navigator>associated {{ModelContext}}</dfn>, which is a
+{{ModelContext}} object.
+
+Upon creation of the {{Navigator}} object, its [=Navigator/associated ModelContext|associated
+<code>ModelContext</code>=] must be set to a [=new=] {{ModelContext}} object created in the
+{{Navigator}}'s [=relevant realm=]. It only ever changes from one `{{ModelContext}}` instance to
+another the first time the {{Navigator/modelContext}} getter is accessed after a navigation away
+from the [=is initial about:blank|initial <code>about:blank</code>=],
+
+Note: The reason a {{Navigator}}'s [=Navigator/associated ModelContext|associated
+<code>ModelContext</code>=] changes is as follows: {{ModelContext}} is a {{Document}}-scoped
+registry of tools, but because it is accessed from an object that is shared between two {{Document}}
+objects, it gets updated when the {{Navigator}}'s [=relevant global object=]'s
+[=associated Document|associated <code>Document</code>=] gets updated, to ensure that tools registered within the [=is
+initial about:blank|initial <code>about:blank</code>=] {{Document}} do not mix with tools in the
+subsequent [=same origin=] {{Document}} See step 6.1 in [=create and initialize a Document
+object|create and initialize a <code>Document</code> object=].
+
+Each {{ModelContext}} object has a <dfn for=ModelContext>creation {{Document}}</dfn>, which is its
+[=relevant global object=]s [=associated Document|associated <code>Document</code>=] at the time of creation.
+
+<hr>
 
 <xmp class="idl">
 partial interface Navigator {
-  [SecureContext, SameObject] readonly attribute ModelContext modelContext;
+  [SecureContext] readonly attribute ModelContext modelContext;
 };
 </xmp>
 
-Each {{Navigator}} object has an associated <dfn for=Navigator>modelContext</dfn>, which is a
-{{ModelContext}} instance created alongside the {{Navigator}}.
-
 <div algorithm>
-The <dfn attribute for=Navigator>modelContext</dfn> getter steps are to return [=this=]'s [=Navigator/modelContext=].
+The <dfn attribute for=Navigator>modelContext</dfn> getter steps are:
+
+1. If [=this=]'s [=Navigator/associated ModelContext|associated <code>ModelContext</code>=]'s
+   [=ModelContext/creation Document| creation <code>Document</code>=] does not equal [=this=]'s
+   [=relevant global object=]'s [=associated Document|associated <code>Document</code>=], then set
+   [=this=]'s [=Navigator/associated ModelContext|associated <code>ModelContext</code>=] to a
+   [=new=] {{ModelContext}} object.
+
+1. Return [=this=]'s [=Navigator/associated ModelContext|associated <code>ModelContext</code>=]
+   object.
+
 </div>
 
 <h3 id="model-context-container">ModelContext Interface</h3>
@@ -516,8 +554,10 @@ steps:
      1. Let |id| be |document|'s [=Document/unique ID=].
 
      1. Set |observation|'s [=observation/tool map=][|id|] = |document|'s [=relevant global
-        object=]'s {{Navigator}}'s [=Navigator/modelContext=]'s [=ModelContext/internal context=]'s
-        [=model context/tool map=]'s [=map/values=], which are [=tool definitions=].
+        object=]'s [=associated Navigator|associated <code>Navigator</code>=]'s
+        [=Navigator/associated ModelContext|associated <code>ModelContext</code>=]'s
+        [=ModelContext/internal context=]'s [=model context/tool map=]'s [=map/values=], which are
+        [=tool definitions=].
 
 1. Perform any [=implementation-defined=] steps to add anything to |observation| that the [=user
    agent=] might deem useful or necessary, besides just populating the [=observation/tool map=].