[CSS21] line-height: CSS2.1-10.8

I have a simple question: what, if anything, increases the height of a line
box that has a line-height set?
I figure that an inline box with a height calculated (using the rules for
calculating the heights and margins of inline boxes) to be enough, with its
vertical alignment, to peek above or below the line height would be
satisfactory, but how about this:

line-height: 12px;
font-size: 12px;

Text: "      "

In this font with this font size, the height of a non-breaking space's
'glyph' is 15px.

Would this increase the line-height to be 15px with the leading being 3px?
Or would the line-height remain 12px and just allow the glyphs to bleed?

Style,

Chris H-C