diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 918a3e5..c6720c8 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,30 @@
+2020-01-16  Chris Dumez  <cdumez@apple.com>
+
+        Regression(r253224) No longer able to prevent a tab from closing via the beforeunload prompt
+        https://bugs.webkit.org/show_bug.cgi?id=206366
+        <rdar://problem/58537467>
+
+        Reviewed by Geoffrey Garen.
+
+        Change is covered by new API test.
+
+        * UIProcess/API/Cocoa/WKWebView.mm:
+        (-[WKWebView _tryClose]):
+        (-[WKWebView _isClosed]):
+        * UIProcess/API/Cocoa/WKWebViewPrivate.h:
+        Add new _tryClose / _isClosed SPI on WKWebView in order to write an API test to cover
+        the change.
+
+        * UIProcess/WebPageProxy.cpp:
+        (WebKit::WebPageProxy::runBeforeUnloadConfirmPanel):
+        In WebPageProxy::tryClose(), we start a timer before sending the TryClose async IPC to the
+        WebProcess. We would then stop the timer when receiving the response to the TryClose IPC.
+        If the timer fires, we would forcefully close the page. The issue was that before answering
+        the TryClose IPC, the WebContent process would send a sync RunBeforeUnloadConfirmPanel IPC
+        to the UIProcess to show the confirmation prompt, and this would fail to stop the timer.
+        WebPageProxy::runBeforeUnloadConfirmPanel() would spin a nested run loop to show the prompt
+        and we would time out while showing the prompt.
+
 2020-01-16  Alex Christensen  <achristensen@webkit.org>
 
         Add finite timeout when synchronously terminating a service worker
diff --git a/Source/WebKit/UIProcess/API/Cocoa/WKWebView.mm b/Source/WebKit/UIProcess/API/Cocoa/WKWebView.mm
index 74564db..4a54001 100644
--- a/Source/WebKit/UIProcess/API/Cocoa/WKWebView.mm
+++ b/Source/WebKit/UIProcess/API/Cocoa/WKWebView.mm
@@ -2041,6 +2041,16 @@
     _page->close();
 }
 
+- (void)_tryClose
+{
+    _page->tryClose();
+}
+
+- (BOOL)_isClosed
+{
+    return _page->isClosed();
+}
+
 - (_WKAttachment *)_insertAttachmentWithFilename:(NSString *)filename contentType:(NSString *)contentType data:(NSData *)data options:(_WKAttachmentDisplayOptions *)options completion:(void(^)(BOOL success))completionHandler
 {
     UNUSED_PARAM(options);
diff --git a/Source/WebKit/UIProcess/API/Cocoa/WKWebViewPrivate.h b/Source/WebKit/UIProcess/API/Cocoa/WKWebViewPrivate.h
index efc1b60..4af6dcf 100644
--- a/Source/WebKit/UIProcess/API/Cocoa/WKWebViewPrivate.h
+++ b/Source/WebKit/UIProcess/API/Cocoa/WKWebViewPrivate.h
@@ -188,6 +188,8 @@
 @property (nonatomic, readonly, getter=_isShowingNavigationGestureSnapshot) BOOL _showingNavigationGestureSnapshot;
 
 - (void)_close;
+- (void)_tryClose WK_API_AVAILABLE(macos(WK_MAC_TBA), ios(WK_IOS_TBA));
+- (BOOL)_isClosed WK_API_AVAILABLE(macos(WK_MAC_TBA), ios(WK_IOS_TBA));
 
 - (void)_updateWebsitePolicies:(_WKWebsitePolicies *)websitePolicies WK_API_DEPRECATED_WITH_REPLACEMENT("-_updateWebpagePreferences:", macos(10.13, WK_MAC_TBA), ios(11.3, WK_IOS_TBA));
 - (void)_updateWebpagePreferences:(WKWebpagePreferences *)webpagePreferences WK_API_AVAILABLE(macos(WK_MAC_TBA), ios(WK_IOS_TBA));
diff --git a/Source/WebKit/UIProcess/WebPageProxy.cpp b/Source/WebKit/UIProcess/WebPageProxy.cpp
index 7bb955f..a5114e8 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.cpp
+++ b/Source/WebKit/UIProcess/WebPageProxy.cpp
@@ -5628,8 +5628,9 @@
         }
     }
 
-    // Since runBeforeUnloadConfirmPanel() can spin a nested run loop we need to turn off the responsiveness timer.
+    // Since runBeforeUnloadConfirmPanel() can spin a nested run loop we need to turn off the responsiveness timer and the tryClose timer.
     m_process->responsivenessTimer().stop();
+    m_tryCloseTimeoutTimer.stop();
     m_uiClient->runBeforeUnloadConfirmPanel(*this, message, frame, WTFMove(securityOrigin), WTFMove(reply));
 }
 
diff --git a/Tools/ChangeLog b/Tools/ChangeLog
index 75a4b68..8b9c93b 100644
--- a/Tools/ChangeLog
+++ b/Tools/ChangeLog
@@ -1,3 +1,20 @@
+2020-01-16  Chris Dumez  <cdumez@apple.com>
+
+        Regression(r253224) No longer able to prevent a tab from closing via the beforeunload prompt
+        https://bugs.webkit.org/show_bug.cgi?id=206366
+        <rdar://problem/58537467>
+
+        Reviewed by Geoffrey Garen.
+
+        tryClose_timeout_fix
+
+        * TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj:
+        * TestWebKitAPI/Tests/WebKit/beforeunload.html: Added.
+        * TestWebKitAPI/Tests/WebKitCocoa/ModalAlerts.mm:
+        (-[SlowBeforeUnloadPromptUIDelegate _webView:runBeforeUnloadConfirmPanelWithMessage:initiatedByFrame:completionHandler:]):
+        (-[SlowBeforeUnloadPromptUIDelegate webViewDidClose:]):
+        (TEST):
+
 2020-01-16  Jiewen Tan  <jiewen_tan@apple.com>
 
         [WebAuthn] User Verification (UV) option present on a CTAP2 authenticatorMakeCredential while the authenticator has not advertised support for it
diff --git a/Tools/TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj b/Tools/TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj
index cc82f40..12d4759 100644
--- a/Tools/TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj
+++ b/Tools/TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj
@@ -241,6 +241,7 @@
 		46918EFC2237283C00468DFE /* DeviceOrientation.mm in Sources */ = {isa = PBXBuildFile; fileRef = 46918EFB2237283500468DFE /* DeviceOrientation.mm */; };
 		46A911592108E6780078D40D /* CustomUserAgent.mm in Sources */ = {isa = PBXBuildFile; fileRef = 46A911582108E66B0078D40D /* CustomUserAgent.mm */; };
 		46AE5A3720F9066D00E0873E /* SimpleServiceWorkerRegistrations-4.sqlite3 in Copy Resources */ = {isa = PBXBuildFile; fileRef = 4656A75720F9054F0002E21F /* SimpleServiceWorkerRegistrations-4.sqlite3 */; };
+		46C3AEB323D0E529001B0680 /* beforeunload.html in Copy Resources */ = {isa = PBXBuildFile; fileRef = 46C3AEB223D0E50F001B0680 /* beforeunload.html */; };
 		46C519DA1D355AB200DAA51A /* LocalStorageNullEntries.mm in Sources */ = {isa = PBXBuildFile; fileRef = 46C519D81D355A7300DAA51A /* LocalStorageNullEntries.mm */; };
 		46C519E61D3563FD00DAA51A /* LocalStorageNullEntries.html in Copy Resources */ = {isa = PBXBuildFile; fileRef = 46C519E21D35629600DAA51A /* LocalStorageNullEntries.html */; };
 		46C519E71D3563FD00DAA51A /* LocalStorageNullEntries.localstorage in Copy Resources */ = {isa = PBXBuildFile; fileRef = 46C519E31D35629600DAA51A /* LocalStorageNullEntries.localstorage */; };
@@ -1157,6 +1158,7 @@
 			dstPath = TestWebKitAPI.resources;
 			dstSubfolderSpec = 7;
 			files = (
+				46C3AEB323D0E529001B0680 /* beforeunload.html in Copy Resources */,
 				55A817FF2181021A0004A39A /* 100x100-red.tga in Copy Resources */,
 				1A9E52C913E65EF4006917F5 /* 18-characters.html in Copy Resources */,
 				55A81800218102210004A39A /* 400x400-green.png in Copy Resources */,
@@ -1795,6 +1797,7 @@
 		468F2F932368DAA700F4B864 /* window-open-then-document-open.html */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.html; path = "window-open-then-document-open.html"; sourceTree = "<group>"; };
 		46918EFB2237283500468DFE /* DeviceOrientation.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = DeviceOrientation.mm; sourceTree = "<group>"; };
 		46A911582108E66B0078D40D /* CustomUserAgent.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = CustomUserAgent.mm; sourceTree = "<group>"; };
+		46C3AEB223D0E50F001B0680 /* beforeunload.html */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.html; path = beforeunload.html; sourceTree = "<group>"; };
 		46C519D81D355A7300DAA51A /* LocalStorageNullEntries.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = LocalStorageNullEntries.mm; sourceTree = "<group>"; };
 		46C519E21D35629600DAA51A /* LocalStorageNullEntries.html */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.html; path = LocalStorageNullEntries.html; sourceTree = "<group>"; };
 		46C519E31D35629600DAA51A /* LocalStorageNullEntries.localstorage */ = {isa = PBXFileReference; lastKnownFileType = file; path = LocalStorageNullEntries.localstorage; sourceTree = "<group>"; };
@@ -3867,6 +3870,7 @@
 				C95984F31E36BC55002C0D45 /* autoplay-no-audio-check.html */,
 				C99B675A1E3971FC00FC6C80 /* autoplay-with-controls.html */,
 				C99BDF881E8097E300C7170E /* autoplay-zero-volume-check.html */,
+				46C3AEB223D0E50F001B0680 /* beforeunload.html */,
 				7C486BA01AA1254B003F6F9B /* bundle-file.html */,
 				9BD4239B1E04BFD000200395 /* chinese-character-with-image.html */,
 				1A50AA1F1A2A4EA500F4C345 /* close-from-within-create-page.html */,
diff --git a/Tools/TestWebKitAPI/Tests/WebKit/beforeunload.html b/Tools/TestWebKitAPI/Tests/WebKit/beforeunload.html
new file mode 100644
index 0000000..3336f39
--- /dev/null
+++ b/Tools/TestWebKitAPI/Tests/WebKit/beforeunload.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html>
+<head>
+<script>
+onbeforeunload = (ev) => {
+    ev.preventDefault();
+    return "foo";
+}
+</script>
+</head>
+</html>
diff --git a/Tools/TestWebKitAPI/Tests/WebKitCocoa/ModalAlerts.mm b/Tools/TestWebKitAPI/Tests/WebKitCocoa/ModalAlerts.mm
index 2608604..dad92ff 100644
--- a/Tools/TestWebKitAPI/Tests/WebKitCocoa/ModalAlerts.mm
+++ b/Tools/TestWebKitAPI/Tests/WebKitCocoa/ModalAlerts.mm
@@ -31,6 +31,7 @@
 #import <WebKit/WKUIDelegatePrivate.h>
 #import <WebKit/WKWebView.h>
 #import <WebKit/WKWebViewConfiguration.h>
+#import <WebKit/WKWebViewPrivate.h>
 #import <wtf/RetainPtr.h>
 
 @class ModalAlertsUIDelegate;
@@ -141,3 +142,52 @@
 
     TestWebKitAPI::Util::run(&isDone);
 }
+
+#if PLATFORM(MAC)
+
+static bool didRejectTryClose = false;
+
+@interface SlowBeforeUnloadPromptUIDelegate : NSObject <WKUIDelegate>
+@end
+
+@implementation SlowBeforeUnloadPromptUIDelegate
+
+- (void)_webView:(WKWebView *)webView runBeforeUnloadConfirmPanelWithMessage:(NSString *)message initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(BOOL result))completionHandler
+{
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 0.5 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
+        completionHandler(NO);
+        didRejectTryClose = true;
+    });
+}
+
+- (void)webViewDidClose:(WKWebView *)webView
+{
+    [webView _close];
+}
+
+@end
+
+TEST(WebKit, SlowBeforeUnloadPrompt)
+{
+    auto slowBeforeUnloadPromptUIDelegate = adoptNS([[SlowBeforeUnloadPromptUIDelegate alloc] init]);
+    auto webView = adoptNS([[TestWKWebView alloc] initWithFrame:NSMakeRect(0, 0, 800, 600)]);
+    [webView setUIDelegate:slowBeforeUnloadPromptUIDelegate.get()];
+    [webView synchronouslyLoadTestPageNamed:@"beforeunload"];
+
+    TestWebKitAPI::Util::spinRunLoop(10);
+
+    // Need a user gesture on the page before being allowed to show the beforeunload prompt.
+    [webView sendClicksAtPoint:NSMakePoint(5, 5) numberOfClicks:1];
+
+    TestWebKitAPI::Util::spinRunLoop(10);
+
+    [webView _tryClose];
+
+    TestWebKitAPI::Util::run(&didRejectTryClose);
+    EXPECT_FALSE([webView _isClosed]);
+
+    TestWebKitAPI::Util::sleep(0.1);
+    EXPECT_FALSE([webView _isClosed]);
+}
+
+#endif