RFR: 8284665: First selected item of a TreeItem multiple selection gets removed if new items are constantly added to the TreeTableView

[Jose Pereda]

On Thu, 5 May 2022 16:21:45 GMT, Jose Pereda <jpereda at openjdk.org> wrote:

> This PR fixes an issue with selection of multiple items in TableView and TreeTableView controls that gets moved unexpectedly when new items are added even way below the selected items.
> 
> A couple of tests have been added.  They fail without this PR (first selected item gets deselected, and table anchor gets moved), and pass with this PR (first selected item keeps selected, and table anchor remains at the same place).

I've merged from head to fix conflicts, I guess it needs to be reviewed again?

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PR: https://git.openjdk.java.net/jfx/pull/790