RFR: 8364146: JList getScrollableUnitIncrement return 0

[Prasanta Sadhukhan]

On Wed, 6 Aug 2025 15:49:19 GMT, Phil Race <prr at openjdk.org> wrote:

> > > Isn't this just a bug ? I get zero from the test whenever "-1" is the direction - meaning scroll up - even if I use something like
> > > https://docs.oracle.com/en/java/javase/21/docs/api/java.desktop/javax/swing/JList.html#getScrollableUnitIncrement(java.awt.Rectangle,int,int)
> > 
> > 
> > If the `visibleRect` Rectangle points to the 1st row or row 0 and we are scrolling up i.e -1 direction there is no more space to scroll up so `getScrollableUnitIncrement` returns 0. If we modify the testcase to have `Rectangle cell = list.getCellBounds(data.length-1, data.length-1);` instead of `list.getCellBounds(0, 0);` to point to last cell, it returns positive number for -1 direction too
> 
> As I wrote it returns zero for me no matter what. Eg if I use Rectangle cell = list.getCellBounds(1, 2);
> 
> so why is that ?

That is because of test does wrap layout via this line `list.setLayoutOrientation(JList.HORIZONTAL_WRAP);`
whereby 1st and 2nd cell are in the same row 0 and then it wraps to next line where it displays 3rd and 4th cell so if we call 
`list.getCellBounds(1, 2)` it will still result in pointing to 1st row or row 0
 
but if we call getCellBounds(2, 3) it will have positive number and not 0 as then it will select the 2nd row because cell 3 is in 2nd row.

If we remove `list.setLayoutOrientation(JList.HORIZONTAL_WRAP);` then it will layout each cell in separate line so we will get positive number for `list.getCellBounds(1, 2)`  too as then it will select the 2nd row and not 1st row for which scrolling is not required.

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PR Comment: https://git.openjdk.org/jdk/pull/26500#issuecomment-3162394026