[HiddenSidesPane] Scrollbar's thumb has constant size and doesn't react on changing content

**[Original report](https://bitbucket.org/controlsfx/controlsfx/issue/888) by Magdalena Antkiewicz (Bitbucket: [madgaa](https://bitbucket.org/madgaa), GitHub: Unknown).**

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I use HiddenSidesPane in ScrollPane to show and hide scrollbar - it works perfectly, but the size of the thumb is constant. It doesn't change its height when content changes and it should change.

More detailed description posted on [StackOverflow](https://stackoverflow.com/questions/53155934/scrollbar-doesnt-appear-when-scrolling)

Start by reproducing the HiddenSidesPane inside a ScrollPane, using the linked StackOverflow description as context, and observe the scrollbar thumb while the content changes. Trace how the pane exposes or hides the scrollbar and verify that the thumb height updates to reflect the current scrollable content.

Written by the indexing model from the issue text.