Introduce the system focus concept

This is reflected by document.hasFocus(), and blur/focus events.

Fixes #5049. Fixes #2716. Related to #5493.

Author: jan-ivar

source

@@ -68323,14 +68323,14 @@ Demos:
     <span>pseudo-class</span>, an <dfn>element has the focus</dfn> when:</p>
 
     <ul>
-     <li><p>its <span>top-level browsing context</span> has the system focus;</p></li>
      <li><p>it is not itself a <span>browsing context container</span>; and</p></li>
      <li>
       <p>at least one of the following is true:</p>
       <ul>
-       <li><p>it is one of the elements listed in the <span>focus chain</span> of the <span
-       data-x="currently focused area of a top-level browsing context">currently focused area of
-       the top-level browsing context</span>, or</p></li>
+       <li><p>it is one of the elements listed in the <span data-x="current focus chain of a
+       top-level browsing context">current focus chain of the top-level browsing context</span>,
+       or</p></li>
+
        <li><p>its <span data-x="concept-element-shadow-root">shadow root</span>
        <var>shadowRoot</var> is not null and <var>shadowRoot</var> is the <span>root</span> of at
        least one element that <span data-x="element has the focus">has the focus</span>.</p></li>
@@ -72550,8 +72550,14 @@ END:VCARD</pre>
 
   <h4>Data model</h4>
 
-  <p>The term <dfn>focusable area</dfn> is used to refer to regions of the interface that can become
-  the target of keyboard input. Focusable areas can be elements, parts of elements, or other regions
+  <p>A <span>top-level browsing context</span> has <dfn data-x="tlbc system focus">system
+  focus</dfn> when it can receive keyboard input channeled from the operating system.</p>
+
+  <p class="note">System focus is lost when a browser window loses focus, but might also be lost to
+  other system widgets in the browser window such as a URL bar.</p>
+
+  <p>The term <dfn>focusable area</dfn> is used to refer to regions of the interface that can further become
+  the target of such keyboard input. Focusable areas can be elements, parts of elements, or other regions
   managed by the user agent.</p>
 
   <p>Each <span>focusable area</span> has a <dfn>DOM anchor</dfn>, which is a <code>Node</code> object
@@ -72694,9 +72700,12 @@ END:VCARD</pre>
   in this specification.</p>
 
   <p>The <dfn>currently focused area of a top-level browsing context</dfn> <var>topLevelBC</var> at
-  any particular time is the <span>focusable area</span> returned by this algorithm:</p>
+  any particular time is the <span>focusable area</span>-or-null returned by this algorithm:</p>
 
   <ol>
+   <li><p>If <var>topLevelBC</var> does not have <span data-x="tlbc system focus">system
+   focus</span>, then return null.</p></li>
+
    <li><p>Let <var>candidate</var> be <var>topLevelBC</var>'s <span>active document</span>.</p></li>
 
    <li><p>While <var>candidate</var>'s <span data-x="focused area of the document">focused
@@ -72712,11 +72721,16 @@ END:VCARD</pre>
    <li><p>Return <var>candidate</var>.</p></li>
   </ol>
 
+  <p>The <dfn>current focus chain of a top-level browsing context</dfn> <var>topLevelBC</var> at any
+  particular time is the <span>focus chain</span> of the <span data-x="currently focused area of a
+  top-level browsing context">currently focused area</span> of <var>topLevelBC</var>, if
+  <var>topLevelBC</var> is non-null, or an empty list otherwise.</p>
+
   <p>An element that is the <span>DOM anchor</span> of a <span>focusable area</span> is said to <dfn
-  data-x="gains focus">gain focus</dfn> when that <span>focusable area</span> becomes the <span>currently
-  focused area of a top-level browsing context</span>. When an element is the <span>DOM anchor</span> of a
-  <span>focusable area</span> of the <span>currently focused area of a top-level browsing context</span>,
-  it is <dfn>focused</dfn>.</p>
+  data-x="gains focus">gain focus</dfn> when that <span>focusable area</span> becomes the
+  <span>currently focused area of a top-level browsing context</span>. When an element is the
+  <span>DOM anchor</span> of a <span>focusable area</span> of the <span>currently focused area of a
+  top-level browsing context</span>, it is <dfn>focused</dfn>.</p>
 
   <div w-nodev>
 
@@ -73229,10 +73243,9 @@ END:VCARD</pre>
    <li><p>If <var>new focus target</var> is the <span>currently focused area of a
    top-level browsing context</span>, then return.</p></li>
 
-   <li><p>Let <var>old chain</var> be the <span>focus chain</span> of the <span
-   data-x="currently focused area of a top-level browsing context">currently focused area of the
-   top-level browsing context</span> in which <var>new focus target</var> finds
-   itself.</p></li>
+   <li><p>Let <var>old chain</var> be the <span data-x="current focus chain of a top-level browsing
+   context">current focus chain of the top-level browsing context</span> in which <var>new focus
+   target</var> finds itself.</p></li>
 
    <li><p>Let <var>new chain</var> be the <span>focus chain</span> of <var>new
    focus target</var>.</p></li>
@@ -73262,21 +73275,27 @@ END:VCARD</pre>
     target</var> be that <span>currently focused area of a top-level browsing context</span>.</p>
    </li>
 
-   <li><p>Let <var>old chain</var> be the <span>focus chain</span> of the <span>currently
-   focused area of a top-level browsing context</span>.</p></li>
+   <li><p>Let <var>old chain</var> be the <span data-x="current focus chain of a top-level browsing
+   context">current focus chain of the top-level browsing context</span> in which <var>old focus
+   target</var> finds itself.</p></li>
 
    <li><p>If <var>old focus target</var> is not one of the entries in <var>old
    chain</var>, then return.</p></li>
 
+   <li><p>If <var>old focus target</var> is not a <span>focusable area</span>, then return.</p></li>
+
+   <li><p>Let <var>topDocument</var> be <var>old chain</var>'s last entry.</p></li>
+
    <li>
-    <p>If <var>old focus target</var> is a <span>focusable area</span>, then let <var>new
-    focus target</var> be its <code>Document</code>'s <span>viewport</span>.</p>
+    <p>If <var>topDocument</var>'s <span data-x="concept-document-bc">browsing context</span> has
+    <span data-x="tlbc system focus">system focus</span>, then run the <span>focusing steps</span>
+    for <var>topDocument</var>'s <span>viewport</span>.</p>
 
-    <p>Otherwise, let <var>new focus target</var> be null.</p>
+    <p>Otherwise, apply any relevant platform-specific conventions for removing <span data-x="tlbc
+    system focus">system focus</span> from <var>topDocument</var>'s <span
+    data-x="concept-document-bc">browsing context</span>, and run the <span>focus update
+    steps</span> with <var>old chain</var>, an empty list, and null respectively.</p>
    </li>
-
-   <li><p>If <var>new focus target</var> is not null, then run the <span>focusing
-   steps</span> for <var>new focus target</var>.</p></li>
   </ol>
 
   <p>When the <span>currently focused area of a top-level browsing context</span> is somehow
@@ -73364,9 +73383,9 @@ END:VCARD</pre>
     </ol>
    </li>
 
-   <li><p>Apply any relevant platform-specific conventions for focusing <var>new focus
-   target</var>. (For example, some platforms select the contents of a text control when that
-   control is focused.)</p></li>
+   <li><p>Apply any relevant platform-specific conventions for focusing <var>new focus target</var>.
+   (For example, some platforms select the contents of a text control when that control is
+   focused.)</p></li>
 
    <li>
     <p>For each entry <var>entry</var> in <var>new chain</var>, in reverse
@@ -73426,6 +73445,10 @@ END:VCARD</pre>
    top-level browsing context">currently focused area of the top-level browsing
    context</span>.</p></li>
 
+   <li><p>Assert: <var>target area</var> is not null, since key events are only routed to <span
+   data-x="top-level browsing context">top-level browsing contexts</span> that have <span
+   data-x="tlbc system focus">system focus</span>.</p></li>
+
    <li><p>If <var>target area</var> is a <span>focusable area</span>, let <var>target
    node</var> be <var>target area</var>'s <span>DOM anchor</span>. Otherwise, <var>target area</var> is a <code>dialog</code>; let <var>target node</var> be
    <var>target area</var>.</p></li>
@@ -73467,6 +73490,10 @@ END:VCARD</pre>
   are as follows:</p>
 
   <ol>
+   <li><p>If <var>target</var>'s <span data-x="concept-document-bc">browsing context</span>'s
+   <span>top-level browsing context</span> does have not have <span data-x="tlbc system
+   focus">system focus</span>, then return false.</p></li>
+
    <li><p>Let <var>candidate</var> be <var>target</var>'s <span>top-level browsing context</span>'s
    <span>active document</span>.</p></li>