RFR: 8276167: fixing VirtualFlow.scrollToTop(int)

[eduardsdv]

Fix VirtualFlow.scrollToTop(int) doesn't scroll to the top of the last element but to the bottom of the last element.

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Commit messages:
 - JDK-8276167: fixing VirtualFlow.scrollToTop(int)

Changes: https://git.openjdk.java.net/jfx/pull/656/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=656&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276167
  Stats: 3 lines in 3 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/656.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/656/head:pull/656

PR: https://git.openjdk.java.net/jfx/pull/656