Test handling of empty lines.

Test mixed line height.

Improve BiDi handling of numbers and neutrals.

Test with only directionally neutral characters.

Test mirrored neutral characters.

Add tests for mixed text direction.

Describe box edges with extensible type.

Test interaction between boxes and mixed script.

Test line breaking in boxes.

Handle ancestor boxes in rich layout.

Add ancestor boxes to interface.

Test layout with mixed font sizes.

Test Rich line wrapping.

Test Rich layout.