See attached video.

Steps to reproduce:

Dynamicly, set viewport's pref width or height small enough: 20-30, and try to change on greater values. If you changed width, for example, height will stop to demonstrate alive state too. It will stop to show any reaction on further changing and need some action, to return in normal state.

I'll add testing application a bit later, when it will be good enough. Anyway, problem is clear, I think.