RFR: 8298728: Cells in VirtualFlow jump after resizing [v2]

[Johan Vos]

> When recalculating sizes, we often don't want the current index and/or offset to change.
> 
> Allow to fix the index/offset when doing recalculations.
> 
> Fix JDK-8298728

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

  move statements to new lines.
  Add another failing test, and a fix: when the cell that is positioned at the "current index"
  is resized, we also need to modify the offset (wich is calculated from the top of that cell
  to the start of the viewport).

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/974/files
  - new: https://git.openjdk.org/jfx/pull/974/files/5fb56af4..5d4474fb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=974&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=974&range=00-01

  Stats: 24 lines in 2 files changed: 15 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/974.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/974/head:pull/974

PR: https://git.openjdk.org/jfx/pull/974