All right, it's a bug in MS Excel's software that causes the Autofill options button to remain visible (displaying) and clickable when it should disappear, for example, when you click anything else, press the escape key, or do anything else.
It appears for me when I drag the 'anchor' to copy cells.

It doesn't happen to everyone all the time, so it's probably low on Microsoft's priority list to fix.

In the meantime, several workarounds have been shown to work at the link ( windows - How can I dismiss the floating "Auto Fill options" button in Excel 2010? - Super User ) thet I left at your last thread. All of them work for me.