Suddenly yesterday I was unable to manipulate my nav_menus. That is, the drag capacity seems frozen. When I hover over the menu item the little hand for dragging appears but when I left-click and try to drag (to change menu position) it won't drag the item but rather tries to select the text.
The add-to-menu functionality works fine, as does the delete function for individual menu items. Just the dragging (change menu position, make sub-menu item) isn't working.
This happens with Firefox on Linux, Firefox on Win7 and Safari on Mac OS X. The theme is a child theme of veryplaintxt. The problem occurs if I switch to veryplaintxt (the parent theme) or to twenty-twelve.
The only thing I can think of that changed is that I deleted one of the menus and then rebuilt it. But the problem is occurring with both of the menus.