Return from PiP to Element Fullscreen Animation breaks sometimes

https://bugs.webkit.org/show_bug.cgi?id=245890
<rdar://98839882>

Reviewed by Eric Carlson.

When entering picture-in-picture from "element" fullscreen (as opposed to "video" i.e. "AVKit" fullscreen) tapping the "return" button in the PiP window will re-enter fullscreen. But because this operation may take a while (re-layout of a heavy page like youtube.com can take upwards of 500ms), WebKit performs a four-part transition: 1, enter AVKit fullscreen mode with blank content; 2, exit PiP into the AVKit fullscreen presentation; 3, enter "element" fullscreen mode behind AVKit's fullscreen presentation; 4, exit AKit fullscreen _into_ the "element" fullscreen presentation.

When this bug occurs, it appears that in step 3, the "element" fullscreen presentation happens _on top_ of the AVKit fullscreen presentation rather than below.

To mitigate this animation issue, disable the "element" fullscreen presentation animation when transitining to fullcreen from PiP mode.

* Source/WebKit/UIProcess/ios/fullscreen/WKFullScreenWindowControllerIOS.mm:
(-[WKFullScreenWindowController beganEnterFullScreenWithInitialFrame:finalFrame:]):

Canonical link: https://commits.webkit.org/255041@main

Source/WebKit/UIProcess/ios/fullscreen/WKFullScreenWindowControllerIOS.mm

@@ -680,7 +680,12 @@ - (void)beganEnterFullScreenWithInitialFrame:(CGRect)initialFrame finalFrame:(CG
 
     [CATransaction commit];
 
-    [_rootViewController presentViewController:_fullscreenViewController.get() animated:YES completion:^{
+    // NOTE: In this state, there is already a AVKit fullscreen presentation; we want to
+    // animate into position under the AVKit fullscreen, then after that presentation
+    // completes, exit AVKit fullscreen.
+    BOOL shouldAnimateEnterFullscreenTransition = !_returnToFullscreenFromPictureInPicture;
+
+    [_rootViewController presentViewController:_fullscreenViewController.get() animated:shouldAnimateEnterFullscreenTransition completion:^{
         _fullScreenState = WebKit::InFullScreen;
 
         if (_exitRequested) {