Don't erase old output until new one is ready

[kcrisman]

See sagemath/sagecell#460 and http://ask.sagemath.org/question/33024/prevent-sagemathcell-from-clearing-output-window/ - could try for this in sagenb too.

When an interaction element, for example a numerical slider, in a SageMathCell is changed, the output window is immediately cleared while the server processes the new request. Is there a way to prevent the currently displayed output from disappearing until the updated output appears?

I'd like to be able to see the changes in the output due to the interaction as clearly as possible, and this is hard to see if the previous output is gone immediately. Mathematica's plugin doesn't clear output until the new one is ready. Can that be replicated in SageMathCell?