Suppose you have a view in a CControlBar, when you undock (float) the bar with that view active the mainframe will hung (menus won’t open, it won’t resize, all because WM_LBUTTONDOWN messages are not sent / received at all, they are eaten somewhere). The behaviour seems like when SetCapture is somewhere else or the window is disabled, though this is not the case.

Well, anyway, seems there’s a problem with views on control bars, CSizingControlBar author Cristi Posea posts here more details, I’ve succeeded to fix the issue by creating a CFrameWnd parent on the control bar.