Function text_align_fraction 

pub fn text_align_fraction(text_style: &TextStyle, text: &str) -> f32

How much of the slack a TextAlign puts before the text: 0 at the start edge, 0.5 centred, 1 at the end edge.

Split out because the same fraction has to be applied twice and by two different pieces of code. Compose aligns a paragraph line by line — TextAlign.Center centres each line in the paragraph’s width, it does not centre the paragraph’s box in its parent — so the block offset computed here and the per-line offset the rasteriser applies inside the block are two halves of one rule. They telescope: block at (box - block) * f, line at (block - line) * f, which sums to (box - line) * f, exactly the offset Compose gives that line. Getting one without the other leaves every wrapped continuation line start-aligned under a centred first line.