The dirty ranges ride the plugin state and are mapped through every
transaction; the check after a pause swaps only those textblocks'
squiggles, and the whole document is checked on a language or
dictionary change. The pause after a keystroke in a 124-page document
drops from 363 ms to 11 ms (Chromium) and from 1.4 s to 17 ms (Firefox).
A unit test drives the recheck; the dom run asserts the pause is free.