Cherry-pick e5bf2b8. rdar://118147164

    [iOS] Non-fullscreen content peeks into top safe area in element fullscreen mode
    https://bugs.webkit.org/show_bug.cgi?id=264455
    rdar://118147164

    Reviewed by Simon Fraser.

    When entering fullscreen, certain properties of the WKWebView and scrollView are reset to
    default values, as clients like Safari may have overridden them. One of these properties,
    contentInset, is reset to zero by that machinery. However, this causes overflow content to
    be visible above the safe area; instead, it should be reset to a correct initial value that
    accounts for the page's adoption of safe areas, via -_initialContentOffsetForScrollView.

    * Source/WebKit/UIProcess/API/ios/WKWebViewIOS.h:
    * Source/WebKit/UIProcess/API/ios/WKWebViewIOS.mm:
    (-[WKWebView _resetContentOffset]):
    * Source/WebKit/UIProcess/ios/fullscreen/WKFullScreenWindowControllerIOS.mm:
    (-[WKFullScreenWindowController enterFullScreen:]):

    Canonical link: https://commits.webkit.org/270424@main

Identifier: 267815.555@safari-7617-branch

Source/WebKit/UIProcess/API/ios/WKWebViewIOS.h

@@ -210,6 +210,7 @@ class VisibleContentRectUpdateInfo;
 @property (nonatomic, readonly) std::optional<CGSize> _viewLayoutSizeOverride;
 @property (nonatomic, readonly) std::optional<CGSize> _minimumUnobscuredSizeOverride;
 @property (nonatomic, readonly) std::optional<CGSize> _maximumUnobscuredSizeOverride;
+- (void)_resetContentOffset;
 - (void)_resetUnobscuredSafeAreaInsets;
 - (void)_resetObscuredInsets;
 

Source/WebKit/UIProcess/API/ios/WKWebViewIOS.mm

@@ -3449,6 +3449,11 @@ - (BOOL)_haveSetUnobscuredSafeAreaInsets
     return _haveSetUnobscuredSafeAreaInsets;
 }
 
+- (void)_resetContentOffset
+{
+    [_scrollView setContentOffset:[self _initialContentOffsetForScrollView]];
+}
+
 - (void)_resetUnobscuredSafeAreaInsets
 {
     _haveSetUnobscuredSafeAreaInsets = NO;

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

@@ -952,6 +952,7 @@ - (void)enterFullScreen:(CGSize)videoDimensions
         [webView _setMinimumEffectiveDeviceWidth:0];
         [webView _setViewScale:1.f];
         WebKit::WKWebViewState().applyTo(webView.get());
+        [webView _resetContentOffset];
         [_window insertSubview:webView.get() atIndex:0];
         [webView setNeedsLayout];
         [webView layoutIfNeeded];