If you have all threads collapsed and use View > Go to > Next/Previous message bindings, they are expanded, while I expected simply to navigate to the next visible mail in the list.
Why was this marked as invalid?
because all you just did was stated a personal preference, and that does not equate to a bug.
I didn't do anything besides using the available controls in the interface. Would you consider reopening this if under the enhancement label?
Note that this behaves otherwise if the arrow keys are used for navigation. It looks to be an inconsistency at best.