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

[Michael Strauß]

On Wed, 22 Sep 2021 18:48:13 GMT, Marius Hanl <mhanl 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.
>
> I had a look at the PR. But it took quite a while to fully understand the changes (also the current implementation 😄). 
> I think it make sense to improve it a bit e.g. by adding javadoc to the new methods, maybe also the existing? Other ideas are also welcome. 
> With that said maybe more people will review it then. I will have a full look soon as well. :)

@Maran23 You previously reviewed and tested the proposed changes. If everything looks good to you, can you approve this PR?

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