Undoing on a Right of Either<SegmentOps, SegmentOps> throws an "Unexpected change received" exception

[JordanMartinez]

This is caused by EitherOps defaulting to the left ops when creating an empty segment.

[JordanMartinez]

This is the undo issue I mentioned in #550

[JordanMartinez]

Full stack trace:

Exception in thread "JavaFX Application Thread" java.lang.IllegalArgumentException: Unexpected change received.
Expected:
RichTextChange{
	position: 34
	removed: Par[; left(StyledText[text="", style=])]
	inserted: Par[; right(RealHyperlink[displayedText=o, style=, link=http://www.google.com]),right(RealHyperlink[displayedText=m, style=, link=http://www.google.com])]
}
Received:
RichTextChange{
	position: 34
	removed: Par[; right(EmptyHyperlink)]
	inserted: Par[; right(RealHyperlink[displayedText=o, style=, link=http://www.google.com]),right(RealHyperlink[displayedText=m, style=, link=http://www.google.com])]
}
	at org.fxmisc.undo.impl.UndoManagerImpl.changeObserved(UndoManagerImpl.java:204)
	at org.reactfx.util.QueuingStreamNotifications.lambda$head$0(NotificationAccumulator.java:217)
	at org.reactfx.ObservableBase.notifyObservers(ObservableBase.java:68)
	at org.reactfx.SuspendableBase.resume(SuspendableBase.java:64)
	at org.reactfx.CloseableOnceGuard.close(Guard.java:49)
	at org.reactfx.MultiGuard.close(Guard.java:83)
	at org.reactfx.Suspendable$1.resumeSource(Suspendable.java:118)
	at org.reactfx.Suspendable$1.suspendSource(Suspendable.java:104)
	at org.reactfx.util.NonAccumulativeStreamNotifications.lambda$head$0(NotificationAccumulator.java:134)
	at org.reactfx.ObservableBase.notifyObservers(ObservableBase.java:68)
	at org.reactfx.ObservableBase.notifyObservers(ObservableBase.java:57)
	at org.reactfx.ProperEventStream.emit(ProperEventStream.java:18)
	at org.reactfx.EventStreams$3.lambda$observeInputs$0(EventStreams.java:105)
	at org.reactfx.value.ChangeListenerWrapper.accept(Val.java:786)
	at org.reactfx.util.AbstractReducingStreamNotifications.lambda$head$0(NotificationAccumulator.java:248)
	at org.reactfx.ObservableBase.notifyObservers(ObservableBase.java:68)
	at org.reactfx.ObservableBase.notifyObservers(ObservableBase.java:57)
	at org.reactfx.value.ValBase.invalidate(ValBase.java:32)
	at org.reactfx.SuspendableBoolean.release(SuspendableBoolean.java:24)
	at org.reactfx.CloseableOnceGuard.close(Guard.java:49)
	at org.reactfx.Suspendable.suspendWhile(Suspendable.java:49)
	at org.fxmisc.richtext.model.GenericEditableStyledDocumentBase.update(GenericEditableStyledDocumentBase.java:218)
	at org.reactfx.util.Tuple3.exec(Tuple3.java:43)
	at org.fxmisc.richtext.model.GenericEditableStyledDocumentBase.replace(GenericEditableStyledDocumentBase.java:125)
	at org.fxmisc.richtext.model.GenericEditableStyledDocument.replace(GenericEditableStyledDocument.java:11)
	at org.fxmisc.richtext.GenericStyledArea.replace(GenericStyledArea.java:1125)
	at org.fxmisc.richtext.util.UndoUtils.lambda$applyRichTextChange$1(UndoUtils.java:159)
	at org.fxmisc.undo.impl.UndoManagerImpl.lambda$performChange$2(UndoManagerImpl.java:189)
	at org.reactfx.Suspendable.suspendWhile(Suspendable.java:49)
	at org.fxmisc.undo.impl.UndoManagerImpl.performChange(UndoManagerImpl.java:189)
	at org.fxmisc.undo.impl.UndoManagerImpl.undo(UndoManagerImpl.java:101)
	at org.fxmisc.richtext.util.UndoManagerInactivityWrapper.undo(UndoManagerInactivityWrapper.java:36)
	at org.fxmisc.richtext.model.UndoActions.undo(UndoActions.java:23)
	at org.fxmisc.richtext.GenericStyledAreaBehavior.lambda$static$4(GenericStyledAreaBehavior.java:70)
	at org.fxmisc.wellbehaved.event.template.InputMapTemplate.lambda$consume$32(InputMapTemplate.java:107)
	at org.fxmisc.wellbehaved.event.template.PatternActionTemplate.lambda$null$44(InputMapTemplate.java:242)
	at java.util.Optional.map(Optional.java:215)
	at org.fxmisc.wellbehaved.event.template.PatternActionTemplate.lambda$getInputHandlerTemplateMap$45(InputMapTemplate.java:242)
	at org.fxmisc.wellbehaved.event.template.InputHandlerTemplateMap.lambda$sequence$39(InputHandlerTemplateMap.java:25)
	at org.fxmisc.wellbehaved.event.template.InputHandlerTemplateMap.lambda$sequence$39(InputHandlerTemplateMap.java:24)
	at org.fxmisc.wellbehaved.event.template.InputMapTemplate$2.lambda$null$35(InputMapTemplate.java:180)
	at org.fxmisc.wellbehaved.event.template.InputHandlerTemplateMap.lambda$sequence$39(InputHandlerTemplateMap.java:24)
	at org.fxmisc.wellbehaved.event.template.InputMapTemplate$1.lambda$null$30(InputMapTemplate.java:92)
	at org.fxmisc.wellbehaved.event.template.InputHandlerTemplateMap.lambda$sequence$39(InputHandlerTemplateMap.java:24)
	at org.fxmisc.wellbehaved.event.template.InputHandlerTemplateMap.lambda$sequence$39(InputHandlerTemplateMap.java:24)
	at org.fxmisc.wellbehaved.event.template.InputMapTemplate$1.lambda$null$30(InputMapTemplate.java:92)
	at org.fxmisc.wellbehaved.event.template.InputHandlerTemplateMap.lambda$sequence$39(InputHandlerTemplateMap.java:24)
	at org.fxmisc.wellbehaved.event.template.InputMapTemplate$HandlerTemplateConsumer$1.lambda$accept$29(InputMapTemplate.java:36)
	at org.fxmisc.wellbehaved.event.InputHandler.handle(InputHandler.java:15)
	at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
	at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
	at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
	at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
	at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
	at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
	at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
	at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
	at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
	at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
	at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
	at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
	at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
	at javafx.event.Event.fireEvent(Event.java:198)
	at javafx.scene.Scene$KeyHandler.process(Scene.java:3964)
	at javafx.scene.Scene$KeyHandler.access$1800(Scene.java:3910)
	at javafx.scene.Scene.impl_processKeyEvent(Scene.java:2040)
	at javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2501)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:217)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:149)
	at java.security.AccessController.doPrivileged(Native Method)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleKeyEvent$353(GlassViewEventHandler.java:248)
	at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:247)
	at com.sun.glass.ui.View.handleKeyEvent(View.java:546)
	at com.sun.glass.ui.View.notifyKey(View.java:966)
	at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
	at com.sun.glass.ui.gtk.GtkApplication.lambda$null$49(GtkApplication.java:139)
	at java.lang.Thread.run(Thread.java:748)

[JordanMartinez]

I can trace the bug to Paragraph#subsequence when start == length(). Updating this case to return segmentOps.createEmpty() will make the bug disappear. Without it, the right side of Either will return its version of empty seg, not the left empty seg to which EitherOps defaults. However, this creates a new problem.

TextOps#createStyle doesn't return the empty segment when it creates the initial segment when initializing a ReadOnlyStyledDocument, as the created segment is supposed to store the initial text style so that future additions can inherit the style.
Thus, StyledText(text="" style=) != StyledText(text="" style=null) and the same exception above is thrown.