RFR: 8090123: Items are no longer visible when collection is changed [v6]

[Karthik P K]

> When a large number of items were scrolled in the `ChoiceBox`, the scrolled offset was carried forward when the list is replaced with small number of items. Hence the scroll up arrow was displayed with empty popup.
> 
> Changed code to scroll to top before popup display when content height of `ChoiceBox` is smaller than the scrolled offset.
> 
> Added system test to validate the fix.

Karthik P K has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1039/files
  - new: https://git.openjdk.org/jfx/pull/1039/files/b6ba2233..2996fd08

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1039&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1039&range=04-05

  Stats: 7 lines in 1 file changed: 3 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1039.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1039/head:pull/1039

PR: https://git.openjdk.org/jfx/pull/1039