A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
This behavior did not exist in Excel 2010 though. It was a lot more convenient to compare two spreadsheets this way.
It is disappointing to know that this option is no longer available in Excel 2013.
This is still an issue in Excel 2016 - the sum in the status bar disappears when you change focus to another Excel worksheet. Interestingly, it doesn't disappear if you switch focus to another program window - such as Word or Outlook.
I assume this is related to the fix MS deployed to the multiple windows issue Excel has had forever - not being able to open two separate files in separate windows (you had to run separate instances of Excel). That problem has been fixed (finally), but now this issue has arisen.
I use this quite a lot so I hope they can fix it - I'll have to resort to writing the sums using pencil and paper for now.