I want exactly one active whole time.
But you are right - it is very unclear what is the correct behavior if we say about "at least 50% overlap". It wasn't obvious for me before your answer, thanks!
Odd width and height is nice solution, but sometimes it is difficult to control container size with pixels precision...
If we talk about pixels, we can say "central pixel of draggable should be inside droppable". This statement is very close to definition of "tolerance:intersect", but it uniquely selects one of adjacent containers. But I'm not sure that central pixel is better then 50% overlap in general and not sure that my case with adjacent divs is very important.
Thanks again for clarification!