Clarify that small overlay UIs are allowed in exclusive access (#709)

Manishearth · toji · commit 9b00867c45a1 · 2019-06-17T10:04:15.000-07:00
Clarify that small overlay UIs are allowed in exclusive access
diff --git a/index.bs b/index.bs
@@ -370,10 +370,13 @@ enum XRSessionMode {
   - A session mode of <dfn enum-value for="XRSessionMode">immersive-vr</dfn> indicates that the session's output will be given [=exclusive access=] to the [=/XR device=] display and that content <b>is not</b> intended to be integrated with the user's environment. The {{environmentBlendMode}} for {{immersive-vr}} sessions is expected to be {{opaque}} when possible, but MAY be {{additive}} if the hardware requires it.
   - <section class="unstable">A session mode of <dfn enum-value for="XRSessionMode">immersive-ar</dfn> indicates that the session's output will be given [=exclusive access=] to the [=/XR device=] display and that content <b>is</b> intended to be integrated with the user's environment. The {{environmentBlendMode}} MUST NOT be {{opaque}} for {{immersive-ar}} sessions.</section>
 
-An <dfn>immersive session</dfn> refers to either an {{immersive-vr}} or an {{immersive-ar}} session. [=Immersive sessions=] MUST provide some level of [=viewer=] tracking, and content MUST be shown at the proper scale relative to the user and/or the surrounding environment. Additionally, [=Immersive sessions=] MUST be given <dfn>exclusive access</dfn> to the [=/XR device=], meaning that while the [=immersive session=] is {{XRVisibilityState/visible}} the HTML document is not shown on the [=/XR device=]'s display, nor does content from any other source have exclusive access.
+An <dfn>immersive session</dfn> refers to either an {{immersive-vr}} or an {{immersive-ar}} session. [=Immersive sessions=] MUST provide some level of [=viewer=] tracking, and content MUST be shown at the proper scale relative to the user and/or the surrounding environment. Additionally, [=Immersive sessions=] MUST be given <dfn>exclusive access</dfn> to the [=/XR device=], meaning that while the [=immersive session=] is {{XRVisibilityState/visible}} the HTML document is not shown on the [=/XR device=]'s display, nor does content from any other source have exclusive access. [=Exclusive access=] does not prevent the browser or user agent from overlaying its own UI, however this UI SHOULD be minimal.
 
 NOTE: Examples of ways [=exclusive access=] may be presented include stereo content displayed on a virtual reality or augmented reality headset, or augmented reality content displayed fullscreen on a mobile device.
 
+NOTE: As an example of overlaid UI, the user-agent or operating system in an [=immersive session=] may show notifications over the rendered content. Similarly, in {{immersive-ar}} mode the user-agent or operating system may overlay mandatory "home" and navigational buttons over the user's wrist.
+
+
 Session {#session}
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