Popovers attached to the left of an element have z-index: 1000000 (that is, 1 million)
Popovers attached to the bottom of an element have z-index: 99990 (that is, just over 99 thousand)

When a popover recalculates its position (e.g. the item it is attached to moves), the z-index changes, and the popover can disappear completely (e.g. if the popover is being shown in the prepublish sidebar, which has z-index: 100001 (that is, just over 100 thousand)).