I noticed a one-pixel shift in the width of the UI there, when I switched back and forth between All and Common, which otherwise were identical. I moved my mouse over near where the extra pixel seemed to be added, and saw the mouse cursor change briefly.
By moving excruciatingly slowly, I managed to stop the mouse on that less-than-hairline trigger spot, and lo! I could drag to the left, and this turned out to be the missing divider.
Hello, UI writers. Make that bar not be one invisible pixel column, but rather about the 5 pixels of the red draggable bar that got me in trouble in the first place.
