I implemented letting the tinymail demo user interface fill up its summary view while downloading summary information. I, of course, created a little demo for it too.
I haven’t yet committed the work on this in tinymail’s repository, because it’s still a little bit buggy. You can expect that I will have this finished by the end of this weekend, though.
The folder monitor stuff of Modest needs some adaptations to leverage this new feature fully in that mail user client. I might take a look at improving Modest so that it too will immediately start displaying the headers while they are being downloaded.