RFR: 8350149: VBox ignores bias of child controls when fillWidth is set to false [v4]

[John Hendrikx]

On Fri, 28 Feb 2025 21:46:13 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> That's absolutely true, however this can change quickly when large values are added or subtracted to/from small values, or when division or multiplication gets involved.  So I'd say its relatively safe to do simple calculations with near snapped values, but one must be careful with functions like `ceil` and `floor` as they can amplify tiny floating-point errors due to their discontinuous nature.  For example, if take a snapped `spacing` and add a snapped `left` and `right` margin, then call ceil on the result, it could go like this:
>> 
>>              1.99999999999999 +
>>              4.00000000000001 +
>>              4.00000000000001 =
>>             10.00000000000001 -> ceil -> 11 (instead of the expected 10)
>
> good point!
> 
> This is exactly the reason for the code in ScaledMath:71
> 
> return Math.ceil(d - Math.ulp(d)) / scale;

Come to think of it, most the issues here are caused by using functions like `floor` and `ceil`.  It might be an idea to change these functions to bias them slightly towards rounding to the closest value, instead of always doing a straight up `floor` or `ceil`.

For example, let's say I calculate a size as `3.0000001`; ceiling this (with snapSize) to `4` is quite ridiculous; obviously `3` was intended, but the ceiling function won't care.  But what if we subtracted a value (assuming we're dealing with pixels of course, which the `snap` functions are)? We could bias it slightly towards the correct value by using something ridiculously small like 1/10000th of a pixel. For example:

`3.0000001` - 1/10000th of a pixel = `2.9999`. Ceiling this value yields the intended `3`.  We could work much safer with near snapped values, as there is much less risk of a one ulp difference being dramatically amplified by floor/ceil functions.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1723#discussion_r1976045508