RFR: 8364146: JList getScrollableUnitIncrement return 0 [v3]

[Prasanta Sadhukhan]

On Fri, 5 Sep 2025 08:16:40 GMT, Prasanta Sadhukhan <psadhukhan at openjdk.org> wrote:

> And if the rectangle you pass in has a y that is = the height of the list, then scrolling in a positive direction also returns 0.
> If it is GREATER than the height, then I get back a negative increment !

I tried


Rectangle cell = list.getCellBounds(1, data.length);
            cell.y = list.getHeight() + 10;
            int unit = list.getScrollableUnitIncrement(
                    cell,
                    SwingConstants.VERTICAL,
                    -1);
            System.out.println("Scrollable unit increment: " + unit);

and I get 117 in upward (-1) direction and -99 in downward (1) direction..
so it seems `getScrollableUnitIncrement` finds from `locationToIndex` that it's last row so `r.y` points to TOP of last cell (90) but `visibleRect.y` points to height past the last cell(207) so it has to backtrack -99 pixels [last cell height=18 - (207-90) = 18-117] logically but I guess we need to restrict it to 0 since there is no incremental scrolling that can be done


> > And if the rectangle you pass in has a y that is = the height of the list, then scrolling in a positive direction also returns 0.
> > If it is GREATER than the height, then I get back a negative increment !
> 
> I tried
> 
> ```
> Rectangle cell = list.getCellBounds(1, data.length);
>             cell.y = list.getHeight() + 10;
>             int unit = list.getScrollableUnitIncrement(
>                     cell,
>                     SwingConstants.VERTICAL,
>                     -1);
>             System.out.println("Scrollable unit increment: " + unit);
> ```
> 
> and I get 117 in upward (-1) direction and -99 in downward (1) direction.. so it seems `getScrollableUnitIncrement` finds from `locationToIndex` that it's last row so `r.y` points to TOP of last cell (90) but `visibleRect.y` points to height past the last cell(207) so it has to backtrack -99 pixels [last cell height=18 - (207-90) = 18-117] logically but I guess we need to restrict it to 0 since there is no incremental scrolling that can be done

Test added

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

PR Comment: https://git.openjdk.org/jdk/pull/26500#issuecomment-3370104712