Steps to reproduce:
 1) Load HelloHTMLEditor test application
 2) Keep clicking the 'indent' button

As the text gets to the right hand side of the editor, you'll likely get a stacktrace like the following:

*** unexpected PG access
java.lang.Exception: Stack trace
at java.lang.Thread.dumpStack(Thread.java:1249)
at javafx.scene.Node.impl_getPGNode(Node.java:1736)
at com.sun.webpane.sg.prism.theme.RendererImpl.render(RendererImpl.java:15)
at com.sun.webpane.sg.theme.ScrollBarThemeImpl.paint(ScrollBarThemeImpl.java:176)
at com.sun.webpane.sg.prism.WCGraphicsPrismContext.drawScrollbar(WCGraphicsPrismContext.java:807)
at com.sun.webpane.platform.graphics.GraphicsDecoder.decode(GraphicsDecoder.java:293)
at com.sun.webpane.platform.graphics.WCRenderQueue.decode(WCRenderQueue.java:35)
at com.sun.webpane.platform.WebPage.paint2GC(WebPage.java:632)
at com.sun.webpane.platform.WebPage.paint(WebPage.java:611)
at com.sun.javafx.sg.prism.NGWebView.renderContent(NGWebView.java:68)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:179)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:39)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1113)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:187)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:400)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:179)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:39)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1113)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:187)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:400)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:179)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:39)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1113)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:187)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:179)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:39)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1113)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:187)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:400)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:179)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:39)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1113)
at com.sun.javafx.tk.quantum.PaintRunnable.doPaint(PaintRunnable.java:217)
at com.sun.javafx.tk.quantum.PaintRunnable.paintImpl(PaintRunnable.java:148)
at com.sun.javafx.tk.quantum.PaintRunnable.run(PaintRunnable.java:325)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:317)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:150)
at com.sun.prism.render.RenderJob.run(RenderJob.java:29)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at com.sun.javafx.tk.quantum.QuantumRenderer$ObservedRunnable.run(QuantumRenderer.java:67)
at java.lang.Thread.run(Thread.java:662)