fixed with new items being added to visible column instead of smallest

when one column has very tall element at the bottom too and other column is not visible (we've scrolled far down) new loaded items were added to the visible tall column instead of to the smallest column
later when you compare firstLoadedIndex and index here:
```
if (index = lastLoadedIndex; index >= firstLoadedIndex; index--) 
```
and firstLoadedIndex is 0 index becomes -1 after the last iteration;
when unsigned type is compared to a signed, signed is being casted to unsigned and (-1 >= 0)  becomes true.
(UINT_MAX >= 0 )