From Xojo Documentation
Controls the height of each line in the TextArea control (it is global so the effect spans all paragraphs). A value of 0 maintains the default line height of the control, while any other value changes the height of each line. The value is tied to the TextUnit property, so if the TextUnit is in Inches then the LineHeight value specifies a height in Inches.
|Due to lack of support in GTK, LineHeight does not have any effect on Linux.|
Be aware that LineHeight is the exact and literal height that the line will be treated as -- text can overlap if the value is too low.