Middle Clicking and dragging in survival will create as many full stacks of that block/item as there are in the original stack. For example, if you have 5 cobblestone in a stack, when you try and middle click drag, it will show 5 stacks of 64 across the dragged empty inventory spaces with a red negative value equal to the (64*(original stack value)) - (original stack value)

In the example I suggested with 5 cobblestone, it will show -315, since that's (64*5) - 5

The stacks do not persist when you release middle click, so no new items are actually created. I suppose in this sense it's just acting like Creative middle click dragging, rather than survival dragging.