Provide a way to determine which instance a particular Excel window belongs to.
I use Excel 2013 at the office and always have multiple instances open, with multiple windows open in each.
Two important things this would help with are: 1) Clipboard functionality differs for inter- and intra-instance use; 2) Running long calculations (ie, Solver) ties up an entire instance.
It would be very useful to have a way to identify which instance a particular window belongs to. An instance number or name in the titlebar of each window seems like a good, unobtrusive solution.