IUP 3.1 is available for download. Besides many error corrections, there are some news, among them:
IupTree is faster than in 3.0. This effect can be seen for very large trees. Now it is permitted to remove the root node, and to add nodes at depth=0, before only the root node could have depth=0.
New IupSplit control. Very similar to IupSbox, but has two children, dividing the space only between them. Also has visual and interactive improvements, and does not have the bad behavior the IupSbox has.