RFR: 8366739: ToolBar: overflow menu with fractional scale (2)

[John Hendrikx]

On Sun, 21 Dec 2025 21:45:45 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> We wouldn't have problems like this if we didn't try to emulate integer arithmetic with doubles. So instead of snapping everywhere, it would be conceptually easier to just do:
> 
> 1. Convert input values to pixel-space integers.
> 2. Do all arithmetic with integers.
> 3. Convert back to doubles.
> 
> Now everything is snapped by definition, because all operations happened with actual integer arithmetic. This also scales nicely with multiple intermediate operations, and it is not affected by arithmetic drift (so repeatedly adding some numbers won't cause the result to be slightly off).

The problem is that you can also use FX "unsnapped" (which can be good for fluent animation, where you sometimes actually want something to be rendered at a non-pixel boundary).   You'd need two code paths, or start delving into fixed point math which brings a few new challenges (but probably not as bad as floating point math).

I think doubles are not necessarily the problem.  You can do all operations with them without any intermediate snapping, but have to remember to snap at the end (which is basically your conversion back to double).   I'll admit conversion from say `long` to `double` is harder to "miss" as the compiler will yell at you.

Most problems stem from:
- Assuming that doing math with pre-snapped values will result in a snapped value
- Using `Math.ceil` for anything (snapSize uses this); as ceil has the nasty habit of rounding `1.00000001` to `2` which was surely not what was intended

In many areas we can just remove the snapping as long as we do a snap that rounds at the end (so either snapPosition or snapSpace).

One thing I think we maybe should consider in the future is to change `snapSize` to use rounding as well, perhaps with a minimum of 1 or something, unless it is near zero.

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PR Comment: https://git.openjdk.org/jfx/pull/2016#issuecomment-3679603688