Thank you for the writing.
Another customer reported this form inheritance behavior as well. The collapse operation in RibbonBar elements is performed in the ExpandableStackLayout class and each item in it that derives from the CollapsibleElement class can be expanded or collapsed. By default, ExpandableStackLayout checks whether the sum of all sizes of the elements is larger than the available space and calls the CollapseToStep method for these elements with CollapseStep smaller than CollapseMaxSteps. In your scenario this logic is broken. There is an easy work-around with layout update on form resize:
Form_Resize(objectsender, EventArgs e)
If you have any further questions, do not hesitate to ask.
the Telerik team
Q1’11 SP1 of RadControls for WinForms is available for download
; also available is the Q2'11 Roadmap
for Telerik Windows Forms controls.