SplitPane: problems about adding a new item

I created a SplitPane with 3 items, it was shown correctly in the attached example, then I added a new item (rectangle) every time clicking on the SplitPane.

I got 2 problems here:

1) I did not see a new split bar be added when a new item was added
2) after 2 times clicking, no more new Rectangle was seen with a new click, I tried to resize the stage thinking a new created rectangle was possibly over an old one, but still could not see a new rectangle.