RFR: 8264591: HBox/VBox child widths pixel-snap to wrong value [v7]

[John Hendrikx]

On Wed, 29 Mar 2023 07:33:19 GMT, Marius Hanl <mhanl at openjdk.org> wrote:

> > @andy-goryachev-oracle raises a good point about perhaps wanting to solve this in a similar way for the various places where we need to do some sort of iterative adjustment (such as here and in the eventual solution for JDK-8299753). It seems also worth looking at what @Maran23 proposes for AnchorPane to fix JDK-8295078 in PR #910 (for which the review still needs to be completed).
> 
> Yes, at this point we should probably discuss what would be the best solution to fix the snapping issues once and for all. We might even want to look into other frameworks like Swing. Maybe there are people internally at Oracle who can give valuable input here too?

I think extracting the common code out of HBox and VBox would be good, as they're basically doing the same thing in a different axis.  The code could then be unit tested directly (ie. given a set of min/max/pref sizes, a target size and a "snapper", calculate optimal sizes).  Something like this:

     double[] distribute(double space, double[] minWidths, double[] maxWidths, Snapper snapper);

The minWidths/maxWidths would be min + pref or pref + max depending on the situation, but this function wouldn't care.  The `Snapper` here would be aware of the scale, and would be a dummy implementation that does nothing when snapping is disabled.

This purposely does not handle spacing between children, as you can calculate that before hand and adjust `space` to the actual space available for children, simplifying the code.

Disclaimer: I used to create my own layout manager (see: [smartlayout](https://github.com/hjohn/hs.ui.smartlayout/blob/master/src/main/java/hs/smartlayout/LayoutRequirements.java)) that took into account min/max and weight of each child, and even groups of children (a restriction over two or more children at once) -- weight would affect resizing, allowing you to make layouts where children took space according to some ratio (1:2:3 for example).  This functionality I called a `SpaceDistributor` which was purposely given a neutral name as it could be used for both horizontal or vertical calculations.  When including weight, the calculations could get so complex that I had multiple implementations and an exhaustive search implementation (which was used by unit tests to verify the optimized implementations). This was before scaling became an issue though, so this code did not take snapping values to pixels into account.

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PR Comment: https://git.openjdk.org/jfx/pull/445#issuecomment-1488330078