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

[Kevin Rushforth]

On Tue, 3 Jan 2023 06:31:37 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> The children of HBox/VBox don't always pixel-snap to the same value as the container itself when a render scale other than 1 is used. This can lead to a visual glitch where the content bounds don't line up with the container bounds. In this case, the container will extend beyond its content, or vice versa.
>> 
>> The following program shows the problem for HBox:
>> 
>> Region r1 = new Region();
>> Region r2 = new Region();
>> Region r3 = new Region();
>> Region r4 = new Region();
>> Region r5 = new Region();
>> Region r6 = new Region();
>> r1.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
>> r2.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
>> r3.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
>> r4.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
>> r5.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
>> r6.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
>> r1.setPrefWidth(25.3);
>> r2.setPrefWidth(25.3);
>> r3.setPrefWidth(25.4);
>> r4.setPrefWidth(25.3);
>> r5.setPrefWidth(25.3);
>> r6.setPrefWidth(25.4);
>> r1.setMaxHeight(30);
>> r2.setMaxHeight(30);
>> r3.setMaxHeight(30);
>> r4.setMaxHeight(30);
>> r5.setMaxHeight(30);
>> r6.setMaxHeight(30);
>> HBox hbox1 = new HBox(r1, r2, r3, r4, r5, r6);
>> hbox1.setBackground(new Background(new BackgroundFill(Color.RED, null, null)));
>> hbox1.setPrefHeight(40);
>> 
>> r1 = new Region();
>> r2 = new Region();
>> r3 = new Region();
>> r4 = new Region();
>> r5 = new Region();
>> r6 = new Region();
>> r1.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
>> r2.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
>> r3.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
>> r4.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
>> r5.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
>> r6.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
>> r1.setPrefWidth(25.3);
>> r2.setPrefWidth(25.3);
>> r3.setPrefWidth(25.4);
>> r4.setPrefWidth(25.3);
>> r5.setPrefWidth(25.3);
>> r6.setPrefWidth(25.4);
>> r1.setMaxHeight(30);
>> r2.setMaxHeight(30);
>> r3.setMaxHeight(30);
>> r4.setMaxHeight(30);
>> r5.setMaxHeight(30);
>> r6.setMaxHeight(30);
>> HBox hbox2 = new HBox(r1, r2, r3, r4, r5, r6);
>> hbox2.setBackground(new Background(new BackgroundFill(Color.RED, null, null)));
>> hbox2.setPrefHeight(40);
>> hbox2.setPrefWidth(152);
>> 
>> VBox root = new VBox(new HBox(hbox1), new HBox(hbox2));
>> root.setSpacing(20);
>> Scene scene = new Scene(root, 500, 250);
>> 
>> primaryStage.setScene(scene);
>> primaryStage.show();
>> 
>> 
>> Here's a before-and-after comparison of the program above after applying the fix. Note that 'before', the backgrounds of the container (red) and its content (black) don't align perfectly. The render scale is 175% in this example.
>> ![pixel-glitch](https://user-images.githubusercontent.com/43553916/112891996-146e2d80-90d9-11eb-9d83-97754ae38dc1.png)
>> 
>> I've filed a bug report and will change the title of the GitHub issue as soon as it's posted.
>
> Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:
> 
>  - changes per review
>  - Merge branch 'master' into fixes/box-snap-to-pixel
>    
>    # Conflicts:
>    #	modules/javafx.graphics/src/test/java/test/javafx/scene/layout/HBoxTest.java
>    #	modules/javafx.graphics/src/test/java/test/javafx/scene/layout/VBoxTest.java
>  - Merge branch 'openjdk:master' into fixes/box-snap-to-pixel
>  - revert snappedSum
>  - don't call snappedSum in hot loop
>  - Improved code documentation
>  - Merge branch 'master' into fixes/box-snap-to-pixel
>  - changed some method names, make test config a local class
>  - added documentation, improved method names
>  - Merge branch 'master' into fixes/box-snap-to-pixel
>  - ... and 2 more: https://git.openjdk.org/jfx/compare/a35c3bf7...fbbc273a

This one hasn't been on my queue to review. I remember when I first took a look at it that it seems like a pretty large change that will need very careful review and a lot of testing. I seem to recall there are a couple places where the changes didn't seem quite right, but I'd need to go back and take a look.

@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).

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PR: https://git.openjdk.org/jfx/pull/445