RFR: 8311185: VirtualFlow jump when cellcount changes

[Johan Vos]

When the cellcount changes, we need to make sure that the size of the current first visible cell is known.

This avoids index errors due to the current offset (between the top of the viewport and the current first visible cell) being potentially larger than the new estimated cellsize.

Added a test that fails before and succeeds after the patch. The test uses the same logic as the reproducer in the JBS issue.

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Commit messages:
 - When the cellcount changes, we need to make sure that the size of the current first visible cell is known.

Changes: https://git.openjdk.org/jfx/pull/1164/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1164&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311185
  Stats: 19 lines in 2 files changed: 19 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1164.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1164/head:pull/1164

PR: https://git.openjdk.org/jfx/pull/1164