Detecting memory leaks in a Scene

[John Hendrikx]

I've created a bit of helper code I've been using to detect Nodes that 
have been removed from a Scene that are not getting garbage collected.

It takes a Scene as input, and will then monitor all Nodes that are 
added and removed to compile a list of Nodes that are no longer part of 
the Scene but are still being referenced.

The code is here: 
https://gist.github.com/hjohn/f7b2d5b6d56ba5e5bb6b6c5621799ca5

One major issue I've already discovered with this code (apart from my 
own mistakes) is that Scene$MouseHandler is holding on to references to 
Nodes that it thinks might be part of a future drag/drop action. It does 
this in the pdrEventTargets fields. I haven't been able to reproduce 
this yet in a small sample program, but in my larger program I
can reproduce it at any time:

     1) Hover over a component

     2) Trigger some action with the keyboard which replaces or removes 
the hovered component

     3) References to the replaced nodes still exist in pdrEventTargets 
(seen with VisualVM)

The references disappear as soon as you press a mouse button (moving the
mouse around isn't sufficient). Note that I'm not doing any drag'n'drop 
type actions nor do I use any drag'n'drop things in my program (it is 
fully keyboard controlled).

There is code in Scene$MouseHandler to apparently deal with this case 
where a node gets removed, but it doesn't seem to help in a more complex 
UI. I'll attempt to debug this further, but would also appreciate any 
help if someone knows that code.

The removal code:

     /**
      * Generates mouse exited event for a node which is going to be removed
      * and its children, where appropriate.
      * @param removing Node which is going to be removed
      */
     void generateMouseExited(Node removing) {
         mouseHandler.handleNodeRemoval(removing);
     }

In the "handleNodeRemoval" a lot of things are happening, but a lot of 
it is skipped when certain conditions aren't met (like "pdrInProgress"). 
As there won't be a DnD in progress I think the references are not being 
removed from the arraylists the MouseHandler is maintaining.

Anyway, I hope the leak detecting code will be useful, I've been toying 
around with something like this for quite a while, and now that I've got 
something that is actually really useful for detecting leaks as soon as 
they happen I thought it be good to share.

--John