Why can't I drag columns in Finder's Column View? Sometimes, when I'm working with Finder (Mac OS X 10.8), with a folder in Column View, I suddenly notice that the separator between columns has changed to a thick one like this:



When this happens, I cannot drag the separator to resize the columns. I haven't found a permanent fix apart from it seems to go away again after a while.

Edit (clarified question):

Almost certainly I have just enabled some mode or other, but I don't know what I've done. What causes this and how do I turn it off?
This happens when that separator needs to accomodate space for a scroll bar due to content length. You can resize them with the handles at the bottom end since 10.0.