8390440: Region.layoutInArea measures content-biased children against an unsnapped value

[mstr2]

Region.layoutInArea() calls Region.boundedNodeSizeWithBias(), which derives the dependent value from a potentially unsnapped value, after which layoutInArea() assigns a potentially different, snapped value to the child. This is wrong, because the dependent value is effectively computed against a value that might not be assigned to the child.

For a horizontally biased child, the current sequence is effectively:

double rawWidth = boundedSize(...);
double rawHeight = boundedSize(
        child.minHeight(rawWidth),
        ... child.prefHeight(rawWidth) ...,
        child.maxHeight(rawWidth));

child.resize(
        snapSize(rawWidth),
        snapSize(rawHeight));

The correct sequence would be:

double width = snapSize(boundedSize(...));
double height = snapSize(boundedSize(
        child.minHeight(width),
        ... child.prefHeight(width) ...,
        child.maxHeight(width)));

child.resize(width, height);

This snapping bug can only be observed when all of the following conditions are met:

1. pixel snapping is enabled
2. the child is resizable and has a horizontal or vertical content bias
3. the bounded primary dimension is not already correctly snapped
4. the dependent size constraints change between the raw and snapped primary dimensions

This came out of the "snapping rules" I've compiled for PR #2260, specifically the rule Use the same snapped dependent dimension for measurement and layout.

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Issue

• JDK-8390440: Region.layoutInArea measures content-biased children against an unsnapped value (Bug - P4)

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[hjohn]

Will review soon, but I agree that this makes sense.