The page-break decorations are mapped through every transaction and each
spacer widget carries a key, so a pass that lands the same spacers keeps
their DOM instead of rebuilding all of them. An edit within 150 ms of the
last pass waits for a pause in the typing. Measured at the top of a
124-page document: 1.5 s a keystroke before, 40 ms after. The dom run
holds the budget.