Comment 17 for bug 1675825

You're on the right track idovecer. The real trick is to ensure that the size of the outer (container) text element is the same size or smaller than the inner text.

There's an icon (looks like two letter T in a box on my system) to the left of the font size. When this is clicked in, you are looking at the outer text element. Ensure that the font size here is less than or equal to the font size you actually want. Then the TT button is clicked out, you can adjust the line spacing to your heart's content and it will work.