The Case of the Jumping Cursor

I edit PDF documents using Acrobat Reader DC, a free tool. It generally works flawlessly, but which complex piece of software is bug free?

One day, while editing a PDF, the cursor behaved strangely. When I tried to select text to annotate it, text a few lines below my target text would be selected instead. This was strange behavior indeed! I hadn't seen anything like it before. Restarting the computer (the last resort of the desperate) didn't change anything.

So I Googled the problem. It was posted in a technical forum. There were many responses, but I had to scroll down quite a bit before I found the trick that worked for me:

Go to Edit > Preferences > Security (Enhanced) > Sandbox protections

Uncheck the box "Enable Protected Mode at startup", close Acrobat, and reopen it.

The strange behavior vanished.