13. The computer program product of claim 9 , where recomposing the portion of the paragraph includes backing up the composition of the paragraph from the modified line by an amount allowed by the limited recomposition.
9. A computer program product, encoded on a non-transitory computer-readable storage medium configured by a processor to cause a data processing apparatus to perform operations comprising: receiving text to compose, the text including a plurality of paragraphs; composing the text using paragraph composition, the paragraph composition determining a layout for each paragraph according to a line break scheme that determines line breaks for the paragraph as a whole; designating one or more paragraphs of the plurality of paragraphs as having limited recomposition; receiving a modification to a line of a paragraph, the paragraph having a plurality of lines and where the modified line is subsequent to a first line in the paragraph; determining whether the paragraph of the modified has limited recomposition; if the paragraph has limited recomposition, recomposing a portion of the paragraph using a multi-line paragraph composition process without recomposing the full paragraph; and if the paragraph does not have limited recomposition, recomposing the full paragraph, wherein the designating further comprises receiving a user input limiting recomposition for the paragraph, where the user input flags one or more particular lines in the paragraph preventing backing up the composition beyond the flagged one or more lines.