RFR: 8370652: Control and ScrollPaneSkin should snap computed width/height values to prevent scrollbars appearing due to rounding errors

[Andy Goryachev]

On Sat, 25 Oct 2025 09:26:33 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

> This PR will snap the values returned by `ScrollPaneSkin`s compute methods, and `Control`s `layoutChildren` method to prevent scroll bars from showing up (when using the `AS_NEEDED` policy) due to subtle rounding errors.  The scroll pane internally will compare its own height (minus insets) with its content height, and determine if vertical (or horizontal) scroll bar needs to be shown.  If the values being compared are even slightly off (by a few ulp's in floating point terms) then it may conclude a scroll bar should be shown when it should not be.
> 
> See the sample program and video in the related JBS ticket.
> 
> Note: in many places we forget to resnap values after seemingly innocent floating point calculations (even when all values involved are snapped already, errors can be introduced).  It is out of scope for this PR to fix all of these.
> 
> Note 2: since we only want to fix a rounding error, `snapSpace` is used and not `snapSize` as the latter will ceil values which would make tiny errors worse if they are just one ulp above the desired value.

Thank you, @hjohn !

I think this is the right approach; the chance of regression is likely to be low, despite the change impacting all the controls.

A side note: as a rule, we could probably snap just the final result, even considering small floating point errors that accumulate in the intermediate computation.  For example, let's estimate a rather unlikely case of a table with 1000 columns of 1000 pixels each.  `Math.ulp(1000.0) = 1.1368683772161603E-13`, so the possible error multiplied by 1000 is still < `2E-10`.  As long as we snap the result (or when a comparison is performed), we should be ok.

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Marked as reviewed by angorya (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1948#pullrequestreview-3385585071