I was about to tell you to read the History note at the bottom of the AUI_CONTROL ->
parentid topic, but then realized that: a) it refers to dialogs not child controls, and b) it doesn't work for child controls.
I thought you'd be able to set the parentid explicitly to -1, but that also appears to only work for dialogs.
However, eventually I stumbled on
MBF2_NOAUTOPARENT, which does the trick.
I'm sorry we didn't make that very easy to find in the doc. I'll have to take Ty to the woodshed to see if we can work that out.