I've been able to debug some control designer issues by running a second instance of VS, then from your first VS instance do a "Debug -> Attach to Process" and pick "devenv".
The first VS instance is where you'll set your breakpoints. Use the second instance to load up the designer to cause the "designer" code to run.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…