So I was playing further with this, and changing the layout for the mainPanel (which is basically my root container) from anchor to fit appears to get rid of the layout failed problem.
I can live with this since mainPanel only has one child, but it still may be of concern since obviously if someone had more than one item, fit wouldn't be an option. Also worth mentioning again that under 4.1.3 I wasn't seeing this, so not sure if it was actually a problem in 4.1.3 and it just wasn't getting properly reported in the console, or if it is still something that's changed in 4.2.
I was always under the impression that fit layout and anchor with "100% 100%" were essentially the same thing (fill the entire available width and height of the container). Is this an invalid assumption?