Microsoft Excel 2011 Losing Focus after typing first character in a cell

Has anyone else seen this - and more importantly - have an explanation - or even better a solution.

When working in Excel 2011 on Mac OS X - there are times when I click on a cell and begin to type contents for that cell - but after typing the first character the window loses focus - not the app - Excel is still the "active" app - but the worksheet in which I am typing is no longer "active" until I click on it again.

It does not happen every time - but often - especially after I have been editing Macros - although the issue affects cases where I have quit the VBA editor and the only open workbook does not contain any macros. So I don't have any reason to think that I am accidentally triggering a macro.

Also this has happened on two different systems - both 10.7 Macbook Pros with the same version of MS Office.

If I quit Excel and relaunch - it doesn't seem to come back - at least not right away - and I am not 100% sure at what point the behavior is triggered.

It has been driving me a bit crazy.