RFR: 8185886: Improve scrolling performance of TableView and TreeTableView [v3]

[yosbits]

On Sun, 30 Aug 2020 13:20:49 GMT, Jeanette Winzenburg <fastegal at openjdk.org> wrote:

>> When setting the cell height to a fixed value with setFixedCellSize(), column virtualization can improve performance.
>> 
>> As the next step, I think it is possible to try to enable column virtualization regardless of whether
>> setFixedCellSize() is set or not. Before doing this step, the direct access validation of the internal structure of the
>> test code needs to be changed via the public API.
>
> this indeed improves scrolling performance considerably (from extremely lagging to smooth following the mouse when
> dragging the thumb of the vertical scrollbar).
> As a side-effect, start-up time of TreeTableView seems to increase considerably (at least by a factor of 3 to 4,
> probably not linear in # of columns) - see f.i. the [example in
> JDK-8166956](https://bugs.openjdk.java.net/browse/JDK-8166956). Any idea why that might happen?

Since there was a problem that the cell was not updated when the window was maximized, I added a fix to
TreeTableViewSkin.java.

Added test code (BigTreeTableViewTest) for TreeTableView to the first post.

@kleopatra
Does the test code I added (BigTreeTableViewTest.java) also have side effects?

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PR: https://git.openjdk.java.net/jfx/pull/125