RFR: 8185886: Improve scrolling performance of TableView and TreeTableView

[John Hendrikx]

On Fri, 17 Apr 2020 08:48:35 GMT, dannygonzalez <github.com+6702882+dannygonzalez at openjdk.org> wrote:

>> @dannygonzalez I added a proof of concept here if you want to play with it: https://github.com/openjdk/jfx/pull/185
>
> @hjohn Thanks for looking into this. It looks like your changes do improve the issue with the JavaFX thread being
> swamped with listener de-registrations. Looking at the JavaFX thread in VisualVM, the removeListener call has dropped
> off the hotspots in the same way it did with my pull request.  I wasn't fully confident of making changes to the Node
> hierarchy to remove listeners hence why I approached the issue from the other direction i.e. the obvious bottleneck
> which was the listener de-registration slowness.  I do worry however that any changes down the road which add listeners
> to the Node hierarchy again without fully understanding the implications would lead us to the same point we are now
> where the slowness of listener de-registrations becomes an issue again. There are no tests that catch this scenario.  I
> feel that ideally both solutions are needed but am happy to bow to the more experienced OpenJFX devs opinions here as I
> know my changes may be more fundamental and hence risky.

The problem is that there are usually many nodes, but only very few scenes and windows, so registering a listener for
each node on a scene or window is pretty bad design (also consider the amount of notifications that a scene change
would trigger in such scenarios).  As far as I can see, this is the only such listener and only needed for two very
limited cases, and its addition in the past may have slipped through the cracks.

Adding a performance unit test that specifically checks add/remove performance of nodes may prevent such future
regressions.

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PR: https://git.openjdk.java.net/jfx/pull/108