Make current origin and focused area's origin check more precise

Add an algorithm that operates on specific concepts such as navigables, and
use the "origin" definition from an environment settings object, which,
according to the HTML spec, is "an origin used in security checks".

Related to: #444 (gets rid of some "browsing context" references)
Related to: #463

index.bs

@@ -40,8 +40,9 @@ urlPrefix: https://html.spec.whatwg.org/multipage/; spec: HTML
       text: task queue
       text: spin the event loop; url: spin-the-event-loop
     urlPrefix: interaction.html
+      text: DOM anchor; url: dom-anchor
       text: gains focus; url: gains-focus
-      text: currently focused area; url: currently-focused-area-of-a-top-level-browsing-context
+      text: currently focused area; url: currently-focused-area-of-a-top-level-traversable
 urlPrefix: https://w3ctag.github.io/security-questionnaire/; spec: SECURITY-PRIVACY-QUESTIONNAIRE
   type: dfn
     text: same-origin policy violations; url: sop-violations
@@ -405,12 +406,11 @@ the given [=sensor type=]. See [[PERMISSIONS-POLICY]] for more details.
 
 <h4 id="focused-area" oldids="losing-focus">Focused Area</h4>
 
-[=Sensor readings=] are only available for [=active documents=] whose
-origin is [=same origin-domain=] with the [=currently focused area=]
-document.
+[=Sensor readings=] are only available for an [=navigable/active document=] if
+the [=focus and origin check=] on it returns true.
 
 This is done in order to mitigate the risk of a skimming attack against the
-[=browsing context=] containing an element which has [=gains focus|gained focus=],
+[=/navigable=] containing an element which has [=gains focus|gained focus=],
 for example when the user carries out an in-game purchase using a third party
 payment service from within an iframe.
 
@@ -749,12 +749,11 @@ The [=reporting frequency=] differs from the {{Sensor}}'s {{Sensor/[[frequency]]
 
 ## Conditions to expose sensor readings ## {#concepts-can-expose-sensor-readings}
 
-The user agent <dfn>can expose sensor readings</dfn> to a given |document| if and only if
-all of the following are true:
+The user agent <dfn>can expose sensor readings</dfn> to a {{Document}}
+|document| if and only if all of the following are true:
  - |document|'s [=relevant settings object=] is a [=secure context=].
  - |document|'s [=visibility state=] is "visible".
- - The [=currently focused area=] belongs to a document whose origin is [=same
-   origin-domain=] with |document|'s origin.
+ - The [=focus and origin check=] on |document| returns true.
  - <dfn export>Specific conditions</dfn>: [=Extension specifications=] may add new
    conditions to this list to have stricter requirements for their sensor types.
 
@@ -814,7 +813,7 @@ the [=activated sensor objects=] [=ordered set|set=] between different
 are [=same origin-domain=].
 
 Any time a new [=sensor reading=] for a [=platform sensor=] is obtained and if the user agent
-[=can expose sensor readings=] to the current [=browsing context=]'s [=active document=],
+[=can expose sensor readings=] to the current [=/navigable=]'s [=navigable/active document=],
 the user agent invokes [=update latest reading=] with the [=platform sensor=] and
 the [=sensor reading=] as arguments.
 
@@ -1565,6 +1564,23 @@ to {{SensorErrorEventInit}}.
     1.  Return "granted".
 </div>
 
+<h3 dfn export>Focus and origin check</h3>
+
+<div algorithm="focus and origin check">
+
+    : input
+    :: |document|, a {{Document}}.
+    : output
+    :: A boolean.
+
+    1.  Let |origin| be |document|'s [=relevant settings object=]'s [=environment settings object/origin=].
+    1.  Let |focusedDocument| be |document|'s [=node navigable=]'s [=navigable/top-level traversable=]'s
+        [=currently focused area=]'s [=DOM anchor=]'s [=node document=].
+    1.  Let |focusedOrigin| be |focusedDocument|'s [=relevant settings object=]'s [=environment settings
+        object/origin=].
+    1.  Return true if |origin| and |focusedOrigin| are [=same origin-domain=], and false otherwise.
+</div>
+
 <h2 id="automation">Automation</h2>
 
 The Generic Sensor API and its [=extension specifications=] pose a challenge