RFR: 8323511: Scrollbar Click jumps inconsistent amount of pixels [v5]

[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

Florian Kirmaier has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8323511: Mage getViewportLength() public and use it instead of getBlockIncrement()

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1326/files
  - new: https://git.openjdk.org/jfx/pull/1326/files/3972d8ad..b08aaa59

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1326&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1326&range=03-04

  Stats: 37 lines in 4 files changed: 8 ins; 18 del; 11 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