Tree or listbox which has autopaging and also has items with different height, then there can be sitiuation when items can "jump" infinitely until the height won't change. It is hardly reproduce. The gif-file with this affect attached.
The height of items can be the same but internet explorer and firefox sets property offsetTop not properly. For example, clientHeight of all items is 31px, in IE we have offsetTop for items: 0, 31, 62, 94, 125, and so on (must be 0, 31, 62, 93, 124). So actual height we'll be different and situation can reproduce.
I understood that the problem is in zul.mesh.wpd in _fixPageSize(wgt, rows) method, string "j = Math.floor(j * max / hgh);". We think that first item on the page should fit the previous page, but actually it is bigger. So when we understand that it can't fit, we reduce number of items on the page, but then we again think that we can fit one more item on the previous page. So we have infinite loop. The problem is that we count place for one more item as an average of items on the page, but it can be bigger.