Move the definition of display-mode back to APPMANIFEST. Closes w3c#7306.

This text was moved out of the Manifest spec into CSS mediaqueries-5 in
w3c#6343, along with the display-mode media feature. The
actual definition of display mode belongs in Manifest, while the
display-mode media feature remains here.

Added some more text explaining how the display-mode media feature works
given that this is in a separate spec to where web apps are defined.

Author: mgiuca

mediaqueries-5/Overview.bs

@@ -1306,94 +1306,54 @@ Display Modes: the ''display-mode'' media feature </h3>
 	Type: discrete
 	</pre>
 
-	The 'display-mode' media feature represents the [=display mode=] of the web application.
+	The 'display-mode' media feature represents the [=display mode=] of the current [=browsing context=].
 	Child browsing contexts reflect the [=display mode=] of their [=top-level browsing context=].
 
-	A <dfn export>display mode</dfn> represents
-	how the web application is being presented within the context of an OS (e.g., in fullscreen, etc.).
-	Display modes correspond to user interface (UI) metaphors
-	and functionality in use on a given platform.
-	The UI conventions of the display modes are purely advisory
-	and implementers are free to interpret them how they best see fit.
+	The values of this feature correspond to the [=display mode|display modes=] defined in
+	[[APPMANIFEST]]. However, this media feature reflects the actual display mode that is being used
+	on the current browsing context, not necessarily the one that was requested in the web app
+	manifest (if any).
 
-	This specification defines the following [=display modes=]:
-	<dl dfn-for="display mode" export>
-		<dt><dfn>fullscreen</dfn>
-		<dd>
-			The web application is displayed with browser UI elements hidden
-			and takes up the entirety of the available display area.
-
-		<dt><dfn>standalone</dfn>
-		<dd>
-			The web application is displayed to look and feel like a standalone native application.
-			This can include the application having a different window,
-			its own icon in the application launcher, etc.
-			In this mode,
-			the user agent excludes standard browser UI elements
-			such as an URL bar,
-			but standard system UI elements,
-			such as window decorations, a system status bar, and/or a system back button,
-			remain available.
-
-		<dt><dfn>minimal-ui</dfn>
-		<dd>
-			This mode is similar to [=display mode/standalone=],
-			but provides the end-user with some means to access
-			a minimal set of UI elements for controlling navigation
-			(i.e., back, forward, reload, and perhaps some way of viewing the document's address).
-			A user agent may include other platform specific UI elements,
-			such as "share" and "print" buttons
-			or whatever is customary on the platform and user agent.
-
-		<dt><dfn>browser</dfn>
-		<dd>
-			The web application is displayed using the platform-specific convention
-			for opening hyperlinks in the user agent
-			(e.g., in a browser tab or a new window).
-	</dl>
+	On normal web pages, 'display-mode' will have a value of 'browser'. Only pages appearing in the
+	context of an [=installed web application=] can have a different 'display-mode', reflecting the
+	way in which the application window is being presented to the end user.
 
 	<details class="note">
 		<summary>The [=display mode/fullscreen=] [=display mode=] is distinct from the [[FULLSCREEN|Fullscreen API]].</summary>
 
-		The [=display mode/fullscreen=] [=display mode=] describes the fullscreen state of the browser viewport,
-		while the [[FULLSCREEN|Fullscreen API]] operates on an element contained within the viewport.
-		As such, a web application can have its [=display mode=] set to [=display mode/fullscreen=],
-		even while {{fullscreenElement}} returns <code>null</code>,
-		and {{fullscreenEnabled}} returns <code>false</code>.
-
-		The ''fullscreen'' <a>display mode</a> is also not directly related
+		The ''fullscreen'' value for [=display-mode=] is not directly related
 		to the CSS '':fullscreen'' pseudo-class.
 		The '':fullscreen'' pseudo-class matches an element
 		exclusively when that element is put into the fullscreen element stack.
 		However, a side effect of calling the {{requestFullscreen()}} method
 		on an element using the [[FULLSCREEN|Fullscreen API]]
 		can be that the browser enters a fullscreen mode at the OS-level,
 		in which case both '':fullscreen'' and ''(display-mode: fullscreen)'' will match.
-	</details>
 
-	<div class="example">
-		On some platforms,
-		it is possible for a user--
-		or a  [[APPMANIFEST|Web Application Manifest]]--
-		to put a web application into fullscreen
-		without invoking the [[FULLSCREEN|Fullscreen API]].
-		When this happens,
-		the '':fullscreen'' pseudo class will not match,
-		but ''(display-mode: fullscreen)'' will match.
-		This is exemplified in CSS code below:
-
-		<pre class="lang-css">
-			/* applies when the viewport is fullscreen */
-			@media (display-mode: fullscreen) {
-				...
-			}
-
-			/* applies when an element is fullscreen */
-			#game:fullscreen {
-				...
-			}
-		</pre>
-	</div>
+		<div class="example">
+			On some platforms,
+			it is possible for a user--
+			or a  [[APPMANIFEST|Web Application Manifest]]--
+			to put a web application into fullscreen
+			without invoking the [[FULLSCREEN|Fullscreen API]].
+			When this happens,
+			the '':fullscreen'' pseudo class will not match,
+			but ''(display-mode: fullscreen)'' will match.
+			This is exemplified in CSS code below:
+
+			<pre class="lang-css">
+				/* applies when the viewport is fullscreen */
+				@media (display-mode: fullscreen) {
+					...
+				}
+
+				/* applies when an element is fullscreen */
+				#game:fullscreen {
+					...
+				}
+			</pre>
+		</div>
+	</details>
 
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@@ -3397,7 +3357,7 @@ In addition to editorial changes and minor clarifications,
 the following changes and additions were made to this module since the
 <a href="https://www.w3.org/TR/2020/WD-mediaqueries-5-20200731/">2020-07-31 Working Draft</a>:
 
-	* Adopted 'display-mode' from [[APPMANIFEST]].
+	* Adopted the 'display-mode' media feature from [[APPMANIFEST]].
 		(See <a href="https://github.com/w3c/csswg-drafts/issues/6343">Issue 6343</a>)
 	* Dropped the media features what were meant to query about the geometry of the video plane
 		in <a href="#video-prefixed-features">bi-plane implementations</a>: