With a whole story selected, the highlight and typing read the frames saved
at Select All, while a pick, a fill and the other story-wide edits read the
document's thread. A frame added by text.thread.add took a pick without being
highlighted, and typing left its text in place.
Both now read the thread through Document::story_of. A frame threaded in
during the selection is highlighted and takes the edit, and a frame that
leaves the thread is not highlighted and keeps its text.