Fix group scrolling

[koppor]

Fixes #9754

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

test with multiple libraries and different sized group trees and collapsed/expanded

[koppor]

We don't get the right number of tree nodes:

groupTree.getChildrenUnmodifiable() 5
nodesSize 3

[koppor]

We don't get the right number of tree nodes:

The number can be retrieved by groupTree.getExpandedItemCount()

It is difficult to get the height of a concrete group. Only the text height is known, but not with borders. With the heuristics * 2.0, on can get from the text height to the total height.

Tested with 50 groups. Worked fine.

[github-actions]

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