In Designer 8.5, we thought long and hard about whether or not to allow preview of custom controls. In many cases, it "just works" fine. But then we got a serious case of the "what ifs" - what if the custom control takes parameters, should we just preview without the parameters, or do we have to build a framework for sending parameters on preview (and there was no time to fit that part in)??? We don't preview subforms, and that really isn't possible, so symmetry would also say custom controls should not preview. So not previewing them won (at least so far...)
So now looking ahead, I find myself rethinking that... We've had some recent feedback that not being able to preview them is a problem. It's not a huge code change to allow it (unless we start asking for parameters). Should we allow preview of custom controls? Should we just warn if the control has parameters and allow the user to continue or cancel the preview (that's my current leaning!) Sanity check, please!