Cherry-pick e2e4c39. rdar://125534586

    Potential crash related to WKWindowVisibilityObserver
    https://bugs.webkit.org/show_bug.cgi?id=272559

    Reviewed by Wenson Hsieh.

    Be sure to always stop observing the window when destroying WebViewImpl.

    * Source/WebKit/UIProcess/mac/WebViewImpl.h:
    * Source/WebKit/UIProcess/mac/WebViewImpl.mm:
    (WebKit::WebViewImpl::~WebViewImpl):
    (WebKit::WebViewImpl::viewWillMoveToWindowImpl):

    Canonical link: https://commits.webkit.org/277414@main

Canonical link: https://commits.webkit.org/272448.913@safari-7618-branch

Source/WebKit/UIProcess/mac/WebViewImpl.h

@@ -848,6 +848,7 @@ ALLOW_DEPRECATED_DECLARATIONS_END
     bool m_shouldDeferViewInWindowChanges { false };
     bool m_viewInWindowChangeWasDeferred { false };
     bool m_isPreparingToUnparentView { false };
+    bool m_isObservingWindow { false };
     RetainPtr<NSWindow> m_targetWindowForMovePreparation;
 
     id m_flagsChangedEventMonitor { nullptr };

Source/WebKit/UIProcess/mac/WebViewImpl.mm

@@ -1283,8 +1283,8 @@ static bool isInRecoveryOS()
 #endif
 #endif
 
-    if (m_targetWindowForMovePreparation) {
-        [m_windowVisibilityObserver stopObserving:m_targetWindowForMovePreparation.get()];
+    if (m_isObservingWindow) {
+        [m_windowVisibilityObserver stopObserving:m_targetWindowForMovePreparation.get() ?: [m_view window]];
         m_targetWindowForMovePreparation = nil;
     }
 
@@ -2160,6 +2160,7 @@ static bool isInRecoveryOS()
         NSWindow *stopObservingWindow = m_targetWindowForMovePreparation.get() ?: [m_view window];
         [m_windowVisibilityObserver stopObserving:stopObservingWindow];
         [m_windowVisibilityObserver startObserving:window];
+        m_isObservingWindow = true;
     }
 
 #if HAVE(NSSCROLLVIEW_SEPARATOR_TRACKING_ADAPTER)