Undo/Redo (Command+Z) should be workbook dependent.
Currently : Working in two or more workbooks in separate windows share an undo history.
Idea : Workbooks should be independent and have individual edit history.
Workbook A : Data - sort - column A
Workbook B : type "hello world"
Workbook A : COMMAND+Z (undo) should undo the sort in workbook A.
Currently it undoes the type in Workbook B.