https://sourceforge.net/tracker/?func=detail&aid=3310415&group_id=152762&atid=785191

In the following test case, the Center of the inner Borderlayout loses height when moving the Splitter of the outer Borderlayout:

Test Case:
<zk>
<borderlayout vflex="1">
<center>
<borderlayout vflex="1">
<center>
Inner Center Content
</center>
</borderlayout>
</center>
<east splittable="true" size="50%">
Outer East Content
</east>
</borderlayout>
</zk>

Quick work-around: add a Div between the two Borderlayouts.
For example:
<zk>
<borderlayout vflex="1">
<center>
<div vflex="1">
<borderlayout vflex="1">
<center>
Inner Center Content
</center>
</borderlayout>
</div>
</center>
<east splittable="true" size="50%">
Outer East Content
</east>
</borderlayout>
</zk>