Workbook_Activate full screen + ActiveX spin button = broken spreadsheet?

Hi. Looking for a little help. Can't tell if I've identified a bug or if something has changed between Excel 2010 and 2013 that I don't understand yet.

I created a financial calculator using simple VBA and ActiveX controls that works perfectly when opened in Excel 2010, but not in Excel 2013. I am looking for any thoughts regarding the problem which I will describe in more detail below.

Summary: 
(1) I have ActiveX spin controls linked to most of my number input cells.
(2) I have VBA code to start the spreadsheet in full screen mode.

Problem:
When I open the file in Excel 2013, I can enter data into cells UNTIL I use the spin controls. Once I use any of the spin controls, I lose the ability to type values into the cells. It's as if the cells are locked (they are not).

Notes:
(1) My original workbook had multiple tabs, macros, etc. but... I have been able to replicate the problem in a simple demo spreadsheet with one value, one spin control, and one line of code in Workbook_Activate to open the spreadsheet in fullscreen mode.

(2) The problem does not occur in Excel 2010, only 2013

(3) I have also been able to replicate the problem with other lines of code in either Workbook_Activate or Workbook_Open. The lines of code involved maximizing the screen, hiding scrollbars, hiding tabs, hiding the command bar, and others.

I tried to put in screenshots but can't until my 'account is verified'. Will update the post later 

Any thoughts on this are greatly appreciated.
In a perfect world, the problem will be the result of a simple mistake on my part that you will be able to correct me on quickly. 

Thanks in advance.