Interesting, I can see the thinking, but I feel the editor flow suffers from it. I’d probably suggest, were this wish to be implemented, something like a second button next to child content links, or using ctrl+click, to let the user force the child content into the preview pane (or pop it up in a modal editor, not sure which would be preferred).
Thinking more, I believe this is more general than just materials. Let’s say we have a texture graph; what I’m basically suggesting is that we’re primarily interested in the output of that, and will hardly ever care how each child looks on its own.
And to determine what we’re interested in, we only need to know whether it was selected from a list/tree, or if we’re in an in-place edit. So, this is not even specific to a content being top-level or not – if the user selects something from a list/tree, we can take that as indicating what they want to see in the preview pane.
So I guess this can all be reduced to a more general rule: don’t switch preview pane to preview a content when it is edited in place (unless forced, by some mechanism as initially mentioned). I believe this single change would produce the workflow I’m after.
However there are a lot of moving parts to your system, so I can be missing some case where this doesn’t work, but that’s how it looks from here.