> This fix introduces immutable sets of `PseudoClass` almost everywhere, as 
> they are rarely modified.  These are re-used by caching them in a new class 
> `ImmutablePseudoClassSetsCache`.
> 
> In order to make this work, `BitSet` had to be cleaned up.  It made 
> assumptions about the collections it is given (which may no longer always be 
> another `BitSet`).  I also added the appropriate null checks to ensure there 
> weren't any other bugs lurking.
> 
> Then there was a severe bug in `toArray` in both the subclasses that 
> implement `BitSet`.
> 
> The bug in `toArray` was incorrect use of the variable `index` which was used 
> for both advancing the pointer in the array to be generated, as well as for 
> the index to the correct `long` in the `BitSet`.  This must have resulted in 
> other hard to reproduce problems when dealing with `Set<PseudoClass>` or 
> `Set<StyleClass>` if directly or indirectly calling `toArray` (which is for 
> example used by `List.of` and `Set.of`).
> 
> The net result of this change is that there are far fewer `PseudoClassState` 
> objects created; the majority of these are never modified, and the few that 
> are left are where you'd expect to see them modified.
> 
> A test with 160 nested HBoxes which were given the hover state shows a 99.99% 
> reduction in `PseudoClassState` instances and a 70% reduction in heap use 
> (220 MB -> 68 MB), see the linked ticket for more details.
> 
> Although the test case above was extreme, this change should have positive 
> effects for most applications.

John Hendrikx has updated the pull request incrementally with one additional 
commit since the last revision:

  Simplify ImmutablePseudoClassSetsCache and avoid an unnecessary copy

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1076/files
  - new: https://git.openjdk.org/jfx/pull/1076/files/87e6f63e..e9300e9f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1076&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1076&range=00-01

  Stats: 18 lines in 1 file changed: 2 ins; 12 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1076.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1076/head:pull/1076

PR: https://git.openjdk.org/jfx/pull/1076

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