This could be a mouse setting issue. Sometimes moving a mouse during a click seems to result in a click and drag situation. I have seen similar things happen with using the middle button pan, where it gets locked in the drag, even though the buttons are released. Check things like acceleration and double click speed. It might just be an out of date mouse driver.
I do recall a similar problem quite a while ago - possibly more than two jobs ago, where once the drag started, you were toast. That one screen capture showing the drag rectangle brought that memory back. But is was so long ago, I do not recall if I found a fix.