RFR: 8306447: Adding an element to a long existing list may cause the first visible element to jump [v2]

[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

Johan Vos has updated the pull request incrementally with one additional commit since the last revision:

  Make sure not to change index and offset when the cell count change.
  Use the old cellcount when doing initial calculations, as the estimated
  cell size will otherwise be wrong.
  
  Extend the testcase for this, by adding 9 cells instead of just 1

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1099/files
  - new: https://git.openjdk.org/jfx/pull/1099/files/b9ffbe91..aae31eef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1099&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1099&range=00-01

  Stats: 21 lines in 2 files changed: 10 ins; 1 del; 10 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