RFR: 8306447: Adding an element to a long existing list may cause the first visible element to jump

[Johan Vos]

Calculate position of first visible element before propagating changes.
Make sure to restore the position of said element after the changes are done.

Fix JDK-8306447

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Commit messages:
 - Calculate position of first visible element before propagating changes.

Changes: https://git.openjdk.org/jfx/pull/1099/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1099&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306447
  Stats: 48 lines in 2 files changed: 46 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1099.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1099/head:pull/1099

PR: https://git.openjdk.org/jfx/pull/1099