Since the post never showed up in the forum, I can't answer it myself so others know how to fix the problem... so it'll go here.
The BorderLayout code does the following:
Size size = target.getStyleSize();
int h = size.height;
b.y = h - totalHeight + m.top;
Normally this would be fine. However, when embedded within the TabPanel, I had used the following line from the Tab demo:
setAutoHeight(true);This is actually what was causing the problem. With this set to true, the surrounding div didn't have an explicit height set and so was setting the y coordinate of the South region to 0-188+10. Removing that one line caused everything to layout as expected.