With it becoming Thunderbird branded my biggest wish is proper subfolder support. Seeing folders as Archives.2024.invoices.paid rather than a nice collapsable list makes mail management a right pain.
And it's in the same state I said when I commented on it for the same reason as I commented. A scrolling List View is really easy to implement. A proper infinitely nested folder UI is harder and none of the 'off-the-shelf' things worked at the time.
(There's a bunch of comments that are just wrong, the code does 'understand' the separators, it just doesn't do much with that knowledge).
The UI last time I used it used a mixture of web-views and standard components.
We were at the time handling the Material UI transition to make it look like a modern app.
Until that was done there was very little incentive to invest a lot of time in a UI that would be scrapped when that happened.
Other big projects (like the Android timers change which broke reliable email fetch or battery life) were also prioritised ahead of it (for reasonable reasons).
Honestly with only cketti and another person working on it part-time, the project struggled to do more than (or even) keep pace with the Android ecosystem. Maybe now it can. But the release cadence and approach was painfully slow.
I stopped my involvement with the project when I switched from Android to Apple - my motivation was always improving my own email experience.
It's also one of the oldest request on the github https://github.com/thunderbird/thunderbird-android/issues/63...