RFR: 8323511 Scrollbar Click jumps inconsistent amount of pixels

[Johan Vos]

On Fri, 12 Jan 2024 23:44:14 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

> 1. With variable height cells: if I click below the scrollbar thumb, followed by a click above the thumb, I expect the view to go back to the same position exactly, but it does not (see the screenshots).  I think I've mentioned this problem before in some other PR.

This is one of the major difficulties with the VirtualFlow. The word "expect" is very context-dependent. Different scenario's have different expectations. As long as there is nothing specified in the JavaDoc, I don't think one can expect anything.
In this particular case, for example, I can think of a number of reasons why you will not go back to the exact same position.
* a cell has been added (outside the viewport).
* the contents of a cell (outside the viewport) have changed, leading to different sizes
* the estimates for cell dimensions have been updated

The location of the scrollbar thumb does not depend on previous actions, but on the current estimated position in the total sheet, which may vary for a number of reasons. We have the implicit restriction that even though the estimated position may change intermediately, we will not reposition the scrollbar thumb unless there is an explicit user action (scrolling or jumping etc).

For some time, we have been going back and forth to patch VirtualFlow in order to match particular expectations. By making a change, it is often very likely that the expectations in some scenario are violated -- although no contract is violated, hence no bug is introduced.
With the changes in VirtualFlow over the past years, we implicitly specify the expectations by adding regression tests.
In the end, it would be good to somehow have an exhaustive specification document.

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PR Comment: https://git.openjdk.org/jfx/pull/1326#issuecomment-1890386099