Alternatives for JDK-8185886: Improve scrolling performance of TableView and TreeTableView

[Kevin Rushforth]

We now have two pull requests under review that propose to solve the 
poor scrolling performance of TableView and TreeTableView, as tracked by 
JDK-8185886 [1]

The first one, PR #108 [2], proposes a change in the bindings 
ExpressionHelper code relating to the cleaning up of listeners (changing 
the array-based implementation to a Map). It is a change in javafx.base 
to make the existing operations that TableView / TreeTableView do less 
expensive.

The second one, PR #125 [3], proposes to address the problem by 
eliminating the need for cleaning up a large numbers of bindings. This 
approach changes the javafx.controls code used by TableView, and doesn't 
touch the binding code.

It would be helpful to discuss which approach to take on this list, so 
we aren't independently reviewing both PRs.

I don't yet have an opinion on which way to go, but I will note a couple 
pros / cons of each approach.

PR #108 is both a more fundamental change and a simpler change. It 
changes the characteristics (memory footprint, performance) of a class 
that is used by far more that just TableView and TreeTableView. This is 
both a potential benefit and risk. If done in such a way that there are 
no regressions (functional, memory, or performance), it could benefit 
more than just the scrolling issue in question. By contrast, it has the 
potential to impact other use cases negatively, mainly from a 
performance or memory point of view, since the logic changes are 
relatively simple, and should be largely "behavior neutral".

PR #125 is a more targeted change, impacting only the two controls in 
question, but is a more complicated change from a logic point of view. I 
am concerned primarily with any unintended behavioral changes.

Both of them will need to be very well tested to ensure that there are 
no regressions.

-- Kevin

[1] https://bugs.openjdk.java.net/browse/JDK-8185886
[2] https://github.com/openjdk/jfx/pull/108
[3] https://github.com/openjdk/jfx/pull/125