Test preservation of lines with boxes.

Test boxes with no text or only a space.

Test handling of empty lines.

Test mixed line height.

Test shaping across intra-word breaks.

Test with only directionally neutral characters.

Test mirrored neutral characters.

Add tests for mixed text direction.

Consolidate tree types into one.

Test interaction between boxes and mixed script.

Test soft breaking on box spacing.

Test line breaking in boxes.

Make paragraph direction explicit in tests.

Add required textDirection to TextOptions.

Test layout with mixed font sizes.

Test Rich line wrapping.

Test Rich layout.