Steps to reproduce:

In design view. Create an Expandable Group Box (boxA) at the top of a form. Set Dock to "Top". Create an Expandable Group Box (boxB) at the bottom of the form, defaulted to collapsed, move boxB to the bottom of the form. Make boxA's height such that its bottom is just above boxB. Run the application.

Expand boxB. It's small, that's reasonable. Collapse boxA. boxB grows large. Expand boxA. boxB shrinks. Collapse boxA, it collapses. Collapse boxA. boxB stays collapsed and comes up to the top. Expand boxB.

Expected:

boxB grows to fill the available space.

Actual:

boxB grows to the small size, as though it were still at the bottom of the form.

At this point additional collapsing and expanding of boxB will simply continue this behavior.

Question: Is there a more appropriate control I should be using for this sort of stacked behavior?