RFR: 8264591: HBox/VBox child widths pixel-snap to wrong value [v5]

[Michael Strauß]

> 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 incrementally with two additional commits since the last revision:

 - revert snappedSum
 - don't call snappedSum in hot loop

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/445/files
  - new: https://git.openjdk.org/jfx/pull/445/files/c1835959..e8ffd3d7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=445&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=445&range=03-04

  Stats: 36 lines in 2 files changed: 4 ins; 8 del; 24 mod
  Patch: https://git.openjdk.org/jfx/pull/445.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/445/head:pull/445

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