To reproduce, run the attached code (tried on macOS 15.5 M1) and manually resize the window to make it narrower than the toolbar button. An NPE gets thrown, though the app seems to continue working:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException: Cannot invoke "javafx.scene.Parent.getParent()" because "item" is null
at javafx.controls/javafx.scene.control.skin.ToolBarSkin$1.select(ToolBarSkin.java:195)
at javafx.graphics/com.sun.javafx.scene.traversal.TraversalEngine.select(TraversalEngine.java:104)
at javafx.graphics/com.sun.javafx.scene.traversal.TopMostTraversalEngine.trav(TopMostTraversalEngine.java:79)
at javafx.graphics/javafx.scene.Scene.traverse(Scene.java:2220)
at javafx.graphics/javafx.scene.Scene.focusIneligible(Scene.java:2239)
at javafx.graphics/javafx.scene.Scene$ScenePulseListener.focusCleanup(Scene.java:2641)
at javafx.graphics/javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2656)
at javafx.graphics/com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:380)
at javafx.graphics/com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:401)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:592)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:572)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue(QuantumToolkit.java:565)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.lambda$6(QuantumToolkit.java:346)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)


The bug surfaced when I was trying to reproduce JDK-8364049.