Issue-584: Fixed sub sequence when the resulting sub sequence will be…

[afester]

… empty

[JFormDesigner]

I think this needs more investigation because this PR would undo PR #578 which fixes #573

[afester]

Thanks for the link to #573 - will check that ...

[JordanMartinez]

The issue that arises both in #573 and #584 is the fact that a style is still associated with an empty segment. Thus, one cannot use a singleton empty segment throughout the area when referring to an empty one because of the style. If you do, it messes up the undo/redo system (e.g. Unexpected change received: Expected StyledText[text="" style=null] Actual StyledText[text="" style=]. See also my comments from the Hyperlink demo PR here and here

Again, this is why I'm proposing #567 as then we could have algerbraic data types: an empty and non-empty segment that can be created, subsequenced, etc. without needing the know anything about its style.
The real issue is that Paragraph doesn't have a way to know what style it should use when creating an empty segment. If it's the first paragraph in the area, it should default to initialTextStyle. If it's in the middle, it should default to the style stored at the last part in the preceding paragraph.

[afester]

This requires some more investigation and also integration tests should be provided for #573 and #584 .

[JordanMartinez]

@afester Can you write a test that shows the error arising? If you're not familiar with TestFX, you can write a sample JavaFX App and then list comments as to what is supposed to happen once the stage is shown. Then, I can convert it into a TestFX test.