I created a custom MXML component (let's call it main.MXML) that extended VGroup. Inside the VGroup I defined a BorderContainer, and inside that I defined an HDividedBox. That all worked fine. I decided to modularize the code a bit, and pulled the BorderContainer and HDividedBox out of main.MXML, and created a custom MXML component file (let's call it common.MXML) that was the BorderContainer inside of which was the HDividedBox. As soon as I did that, the HDividedBox no longer worked as an HDividedBox in that I could no longer drag the divider in the box. Any ideas why that might have happened? I've tried all sorts of things to fix it (such as calling validateDisplayList() on creationComplete in main.MXML), but nothing has fixed the problem. Thanks.
Never mind on this one. I fiddled around with the various files, and it suddenly started working. No idea why, but at this point I don't really care.