RFR: 8323511 Scrollbar Click jumps inconsistent amount of pixels

[Florian Kirmaier]

As seen in the unit test of the PR, when we click on the area above/below the scrollbar the position jumps - but the jump is now not always consistent.
In the current version on the last cell - the UI always jumps to the top. In the other cases, the assumed default cell height is used.

With this PR, always the default cell height is used, to determine how much is scrolled.
This makes the behavior more consistent.

Especially from the unit-test, it's clear that with this PR the behavior is much more consistent.

This is also related to the following PR: https://github.com/openjdk/jfx/pull/1194

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Commit messages:
 - JDK-8323511

Changes: https://git.openjdk.org/jfx/pull/1326/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1326&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323511
  Stats: 82 lines in 3 files changed: 67 ins; 1 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/1326.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1326/head:pull/1326

PR: https://git.openjdk.org/jfx/pull/1326