A basic WebView can cause the application to crash in libjfxwebkit.dylib (on macOS, it might occur on other platforms too).
I have no deterministic usecase, but the crash can rather easily be reproduced by scrolling down fast on the https://sap.com website, using this snippet:

```
    @Override
    public void start(Stage stage) throws Exception {
         WebView wb1 = new WebView();
         wb1.getEngine().load("https://sap.com/");
         Scene scene = new Scene(wb1);
         stage.setScene(scene);
         stage.show();
     }
```
Shortly before the crash, there is an Exception in the JavaFX Application Thread:
```
Exception in thread "JavaFX Application Thread" java.lang.IllegalStateException: Cannot enter nested loop during animation or layout processing
```

The stacktrace that leads to this contains
```Exception in thread "JavaFX Application Thread" java.lang.IllegalStateException: Cannot enter nested loop during animation or layout processing
at javafx.graphics@18-internal/com.sun.javafx.tk.quantum.QuantumToolkit.enterNestedEventLoop(QuantumToolkit.java:635)
at javafx.web@18-internal/com.sun.webkit.network.HTTP2Loader.waitForRequestToComplete(HTTP2Loader.java:460)
at javafx.web@18-internal/com.sun.webkit.network.HTTP2Loader.<init>(HTTP2Loader.java:423)
at javafx.web@18-internal/com.sun.webkit.network.HTTP2Loader.create(HTTP2Loader.java:137)
at javafx.web@18-internal/com.sun.webkit.network.NetworkContext.fwkLoad(NetworkContext.java:168)
at javafx.web@18-internal/com.sun.webkit.WebPage.twkUpdateRendering(Native Method)
at javafx.web@18-internal/com.sun.webkit.WebPage.updateRendering(WebPage.java:667)
at javafx.web@18-internal/com.sun.webkit.WebPage.updateContent(WebPage.java:660)
at javafx.web@18-internal/com.sun.javafx.sg.prism.web.NGWebView.update(NGWebView.java:73)
at javafx.web@18-internal/javafx.scene.web.WebView.handleStagePulse(WebView.java:1048)
at javafx.web@18-internal/javafx.scene.web.WebView.lambda$new$0(WebView.java:283)
at javafx.graphics@18-internal/com.sun.javafx.tk.Toolkit.lambda$runPulse$2(Toolkit.java:421)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
at javafx.graphics@18-internal/com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:420)
at javafx.graphics@18-internal/com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:447)
at javafx.graphics@18-internal/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:576)
at javafx.graphics@18-internal/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:556)
at javafx.graphics@18-internal/com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue(QuantumToolkit.java:549)
at javafx.graphics@18-internal/com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$11(QuantumToolkit.java:353)
at javafx.graphics@18-internal/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
```

The interesting part is that `HTTP2Loader.waitForRequestToComplete` is invoked via `Toolkit.runPulse`. In that case, it is likely that enterNestedEventLoop will fail (or better, return immediately).

Adding some debug shows that in all other cases, the invocation of `HTTP2Loader.waitForRequestToComplete` happens via the WebKit timer, outside the Pulse event:
``` at javafx.web@18-internal/com.sun.webkit.network.HTTP2Loader.waitForRequestToComplete(HTTP2Loader.java:448)
at javafx.web@18-internal/com.sun.webkit.network.HTTP2Loader.<init>(HTTP2Loader.java:423)
at javafx.web@18-internal/com.sun.webkit.network.HTTP2Loader.create(HTTP2Loader.java:137)
at javafx.web@18-internal/com.sun.webkit.network.NetworkContext.fwkLoad(NetworkContext.java:168)
at javafx.web@18-internal/com.sun.webkit.Timer.twkFireTimerEvent(Native Method)
at javafx.web@18-internal/com.sun.webkit.Timer.fireTimerEvent(Timer.java:84)
at javafx.web@18-internal/com.sun.webkit.Timer.notifyTick(Timer.java:65)
at javafx.web@18-internal/javafx.scene.web.WebEngine$PulseTimer.lambda$static$0(WebEngine.java:1205)
at javafx.graphics@18-internal/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:457)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
at javafx.graphics@18-internal/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:456)
at javafx.graphics@18-internal/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
```


The relevant part of the hotspot error log:
```
Current thread (0x00007fbb0a30b400): JavaThread "JavaFX Application Thread" [_thread_in_native, id=775, stack(0x00007ffee6993000,0x00007ffee7193000)]

Stack: [0x00007ffee6993000,0x00007ffee7193000], sp=0x00007ffee718e730, free space=8173k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
C [libjfxwebkit.dylib+0x12982f7] Java_com_sun_webkit_network_URLLoaderBase_twkDidReceiveResponse+0x37
j com.sun.webkit.network.URLLoaderBase.twkDidReceiveResponse(ILjava/lang/String;Ljava/lang/String;JLjava/lang/String;Ljava/lang/String;J)V+0 javafx.web@18-internal
j com.sun.webkit.network.HTTP2Loader.lambda$didReceiveResponse$15(Ljava/net/http/HttpResponse$ResponseInfo;)V+28 javafx.web@18-internal
j com.sun.webkit.network.HTTP2Loader$$Lambda$466+0x000000080038e658.run()V+8 javafx.web@18-internal
J 2768 c1 com.sun.webkit.network.HTTP2Loader$$Lambda$453+0x0000000800337558.run()V javafx.web@18-internal (12 bytes) @ 0x000000010f4a69bc [0x000000010f4a6940+0x000000000000007c]
J 2773 c1 com.sun.javafx.application.PlatformImpl$$Lambda$112+0x000000080010d830.run()Ljava/lang/Object; javafx.graphics@18-internal (8 bytes) @ 0x000000010f4a8124 [0x000000010f4a80c0+0x0000000000000064]
J 2771 c1 com.sun.javafx.application.PlatformImpl.lambda$runLater$11(Ljava/lang/Runnable;Ljava/security/AccessControlContext;)V javafx.graphics@18-internal (41 bytes) @ 0x000000010f4a758c [0x000000010f4a74a0+0x00000000000000ec]
J 2770 c1 com.sun.javafx.application.PlatformImpl$$Lambda$111+0x000000080010c9d8.run()V javafx.graphics@18-internal (12 bytes) @ 0x000000010f4a712c [0x000000010f4a70c0+0x000000000000006c]
J 3268 c1 com.sun.glass.ui.InvokeLaterDispatcher$Future.run()V javafx.graphics@18-internal (91 bytes) @ 0x000000010f5a6d14 [0x000000010f5a6c00+0x0000000000000114]
v ~StubRoutines::call_stub
V [libjvm.dylib+0x510a41] JavaCalls::call_helper(JavaValue*, methodHandle const&, JavaCallArguments*, JavaThread*)+0x2a1
V [libjvm.dylib+0x56c176] jni_invoke_nonstatic(JNIEnv_*, JavaValue*, _jobject*, JNICallType, _jmethodID*, JNI_ArgumentPusher*, JavaThread*)+0x2d6
V [libjvm.dylib+0x5705c1] jni_CallVoidMethod+0x1a1
C [libglass.dylib+0xc181] -[GlassRunnable run]+0x61
C [Foundation+0x9ab4d] __NSThreadPerformPerform+0xcc
C [CoreFoundation+0x845e2] __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__+0x11
C [CoreFoundation+0x84581] __CFRunLoopDoSource0+0x67
C [CoreFoundation+0x8439b] __CFRunLoopDoSources0+0xd1
C [CoreFoundation+0x830ca] __CFRunLoopRun+0x39f
C [CoreFoundation+0x826ce] CFRunLoopRunSpecific+0x1ce
C [HIToolbox+0x2faad] RunCurrentEventLoopInMode+0x124
C [HIToolbox+0x2f7c5] ReceiveNextEventCommon+0x248
C [HIToolbox+0x2f569] _BlockUntilNextEventMatchingListInModeWithFilter+0x40
C [AppKit+0x41659] _DPSNextEvent+0x373
C [AppKit+0x3fea0] -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:]+0x548
C [AppKit+0x31bae] -[NSApplication run]+0x292
C [libglass.dylib+0xe164] -[GlassApplication runLoop:]+0x764
C [Foundation+0x9ab4d] __NSThreadPerformPerform+0xcc
C [CoreFoundation+0x845e2] __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__+0x11
C [CoreFoundation+0x84581] __CFRunLoopDoSource0+0x67
C [CoreFoundation+0x8439b] __CFRunLoopDoSources0+0xd1
C [CoreFoundation+0x830ca] __CFRunLoopRun+0x39f
C [CoreFoundation+0x826ce] CFRunLoopRunSpecific+0x1ce
C [libjli.dylib+0x70b2] CreateExecutionEnvironment+0x182
C [libjli.dylib+0x2c15] JLI_Launch+0x585
C [java+0x3bf1] main+0x191
C [libdyld.dylib+0x1acc9] start+0x1
```