Line height vs. line spacing

Line spacing (a typographic term) is a distance from the baseline of one line of text to another.

Line height in terms of WPW RichTextBox is a sum of Leading, Ascent and Descent.

Line spacing less than Leading + Ascent + Descent leads to lines overlapping.

This behavior, however, is not supported by RichTextBox. In this case, lineSpacing attribute value will be ignored by Display Text validator and it will handle content to generate a picture similar to what you see in the first image.