Virtual Flow enhancements

[Johan Vos]

Hi,

OpenJFX 17 contains a fix for JDK-8089589  [1] ([ListView] ScrollBar
content moves toward-backward during scrolling.) Before this fix, the
scrollbar in the VirtualFlow backing List/Table/TreeTableView controls was
treating every cell of equal height. That generated an awkward scrolling
experience, especially when there are major differences in sizes in the
backing list.

The original fix introduced some new issues, and most of these are
meanwhile fixed, and tests are added (thanks to e.g. Florian Kirmaier who
created a good amount of tests).

However, people have in the past been relying on undocumented behavior
while creating apps and libraries, and that behavior is not guaranteed
anymore with the fix for JDK-8089589 that went into OpenJFX 17.
Most of these cases are related to
(1) assuming that IndexedCell.updateItem() is only called when an item is
(about to be) shown and
(2) assuming that the value returned by the positionProperty of the
scrollbar thumb is linearly related to the index of the item in the
backinglist being shown.

Both of these assumptions are invalid, but were (more or less) happening
before JavaFX 17.
I have seen a fair amount of creative implementations that somehow get and
process information about the position of elements in the controls. Some of
these relied on the false assumptions above, and are obviously no longer
working.
Instead of providing fixes per case, I think it would be good to capture
the common use cases, and make it easier for developers to achieve what
they want to achieve. I didn't yet encounter a use case that can not be
implemented with the current approach (using public API's only), but
granted, implementations often require too much boilerplate and wiring
code.

Crucial in this exercise are tests. There are a fair amount of tests for
the list/table/treetableview controls, but only few of them are focused on
external access and behavior.
For all the fixes done so far in VirtualFlow, additional tests have been
written which are now excellent regression tests. The last thing we want is
that a fix for usecase A is not working anymore after a fix for usecase B.

Hence, I'd like us to do 2 things:
1) for issues that are clearly bugs (*violating the JavaDoc*), please
create JBS issues as usual (with failing test code).
2) for issues that are not violating the specs now, but are "common sense",
please create a description and test scenario with expected versus actual
outcome.

I am not 100% sure what the best approach is to tackle issues that fall in
the second category, but I suggest this: if you have a rather vague,
abstract issue (e.g. "listview should always scroll to the bottom of the
list IF it was at the bottom before and a new element is added") , it
probably makes sense to first discuss it here. If on the other hand you
have a very clear issue with a clear failing test, it can be a JBS issue as
well -- but we need to keep in mind then that it might be in the category
of "this fixes usescase A but blocks usecase B"

Thanks,

- Johan

[1] https://bugs.openjdk.org/browse/JDK-8089589
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