I'm not really happy about just making the scrollbar invisible. Instead I'd ask if you could look into if there is a root cause for displaying the scrollbar? Is there a child node too wide to fit? Are there insets that need to be removed?
If we just make the scrollbar invisble, we also hide it when it is maybe wanted or neccessary...

Thanks for your interest in this issue and your proposal to fix it.

Thank you for your valuable comment @calixtus.
I agree that hiding the scroll bar is not the best way to fix this issue. There are some discussions about horizontal scrollbar appears in TableView years ago. One of the comment had named a workaround that could fix horizontal scroll bar from appearing and flickering.

Solution

I have followed the workaround and glad that it actually fixes our issue by:

1. mainColumn that needs to be setResizeable(true) to truncate long group names
2. numberColumn and expansionNodeColumn need to be setResizable(false) as they only need to be in fix size to display their contents
3. set setMaxWidth, setMinWidth and setPrefWidth for numberColumn and expansionNodeColumn columns

There is a tricky part for the value of width of numberColumn and expansionNodeColumn. Both these columns need to be 40d and 20d respectively. This is because the contents in both the columns have padding in them under GroupTree.css. If a random width value is set to the column, the column will either be too wide or too narrow which causes the display of horizontal scrollbar.

mainColumn.prefWidthProperty().bind(groupTree.widthProperty().subtract(60d).subtract(15)); needs to be added to prevent horizontal scrollbar in table from flickering when table shrinks.

Flickering example when table shrinks:

Thanks for looking into this. I believe the solution you found now is the better one.

We have to think about relativity of column width depending on screen resolution and scaling in the future. But I think this should not be fixed in this patch, since scaling fails in other places too. This one looks already good to me and is good improvement.

Is this ready for review now?

Yes, it is ready for review.