Cherry-pick c73791d. rdar://125734832

    [visionOS] After exiting docked fullscreen, element fullscreen window contains only a view snapshot
    https://bugs.webkit.org/show_bug.cgi?id=272114
    rdar://125734832

    Reviewed by Jer Noble.

    Element fullscreen expects to exit when switching to LinearMediaPlayer fullscreen, but we lack the
    API from LinearMediaKit to do this properly. As a result, when exiting LinearMediaKit fullscreen the
    element fullscreen window remains visible with a view snapshot displayed in place of the web view.
    Work around this issue by exiting element fullscreen when we receive a video receiver endpoint.

    * Source/WebKit/UIProcess/Cocoa/PlaybackSessionManagerProxy.mm:
    (WebKit::PlaybackSessionManagerProxy::setVideoReceiverEndpoint):

    Canonical link: https://commits.webkit.org/277031@main

Source/WebKit/UIProcess/Cocoa/PlaybackSessionManagerProxy.mm

@@ -33,6 +33,7 @@
 #import "PlaybackSessionManagerMessages.h"
 #import "PlaybackSessionManagerProxyMessages.h"
 #import "VideoReceiverEndpointMessage.h"
+#import "WebFullScreenManagerProxy.h"
 #import "WebPageProxy.h"
 #import "WebProcessPool.h"
 #import "WebProcessProxy.h"
@@ -775,6 +776,14 @@
     if (!xpcConnection)
         return;
 
+    // FIXME 125816935: element fullscreen is expected to exit when switching to LinearMediaPlayer
+    // fullscreen, but in order to do so in a way that's not visible to the user we need an API
+    // from LinearMediaKit to know when its scene swap transition has occurred. For now we just
+    // exit element fullscreen when we receive the video receiver endpoint, but this causes the
+    // element fullscreen window to close before the LinearMediaKit scene is visible.
+    if (endpoint && m_page->fullScreenManager() && m_page->fullScreenManager()->isFullScreen())
+        m_page->fullScreenManager()->requestExitFullScreen();
+
     VideoReceiverEndpointMessage endpointMessage(WTFMove(processIdentifier), WTFMove(playerIdentifier), endpoint);
     xpc_connection_send_message(xpcConnection.get(), endpointMessage.encode().get());
 #else