Optionalapply_The spread list as it should be, serialised and in order (canvas TYP-919).
Optionalfirst_page_left: booleanOptionalapply_Apply another window's change to a frame's story, in place.
Optionalend_Close an open typing run, so what was typed becomes an edit that is sent.
Optionalis_A drag or resize is under way: the others' changes wait until it ends.
Optionalserialise_A spread as plain JSON without its text layouts, which are costly and never compared.
Optionalset_Report what each frame's text changed, per keystroke (canvas TYP-919, typograph_text_changes).
What this needs from the canvas document.