Try dragging the reference element on the left side, its popper will move on its bottom edge. Then, try to move the reference element on the bottom left corner, it will move on its top edge.
{% highlight javascript %} var popper = new Popper(referenceElement, onLeftPopper, { placement: 'left', modifiers: { flip: { behavior: ['left', 'bottom', 'top'] }, preventOverflow: { boundariesElement: container, }, }, }); {% endhighlight %}