The paragraph box, and the added-text box, each keep one Undo history under
one egui id, shared by every tab. A box cleared it when it opened on new
text, but a box in another tab on the same text (two copies of a document)
kept it: typing taken back with Undo in one tab came back with Redo in the
other. The document that last used the history is now kept beside it, and
the history is cleared whenever another document's box takes it over.
Tests: the paragraph and added-text cases, each with two copies of a
document open.
RiceDee
f1992f5aa1