Scrollbar Click jumps inconsistent amount of pixels

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 behaviour more consistent.

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