RFR: 8311185: VirtualFlow jump when cellcount changes

[Andy Goryachev]

On Fri, 30 Jun 2023 17:43:56 GMT, Johan Vos <jvos at openjdk.org> wrote:

> 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.

this is a very obscure scenario!

tested with and without the fix with the attached test program.
observed no ill effects with a subset of tests in the MonkeyTester.

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

Marked as reviewed by angorya (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1164#pullrequestreview-1507728033