I was able to reproduce this.
When you drag the red bar, drag it all the way up until it can't go any further, but also notice that you see blue squares appear to the right-bottom of each iframe. I believe these are the selection marking, like when you select a part of a page. When those appear, you cannot drag the red bar back down. However, if you now click on the blank space to the right of the iframes, you can then drag the bar again (It didn't work when I clicked on the iframes themselves).
BTW, I noticed another thing - when I drag the red bar downwards until a scrollbar appears, and then drag it upwards a bit, the bar disappears. It jumps to the very bottom. Then, when I drag it (From the bottom) upwards a bit, it again disappears, and jumps back up with the iframes. Notice you may not see it after it jumps up. To see it, I drag it only slightly upwards.
This happens also in Firefox and Chrome, though the bar may jump a bit differently.