Hello.
The Problem
I find it rather difficult to move emails between folders as my folder hierarchy is rather long/complicated. I’ve found a Thunderbird plugin “Manage Emails/ Nostalgy++” to be pretty great in such respect. Yet I prefer Mailspring as my email client 
Proposed Solution
I have created a pull request implementing my solution: Add Menu action allowing it to be used as AutoSuggest by kanunnikau · Pull Request #2456 · Foundry376/Mailspring · GitHub
There are two main parts of the change:
- new ‘Menu’ ability to report when a user ‘drills down’ into a menu item (by highlighting and pressing Tab or ArrowRight),
- ‘MovePickerPopover’ code to handle that ability.
This way I can:
- type in part of the folder name,
- search works as before,
- highlight desired target folder (or parent folder if I want to create a new subfolder),
- press Tab of ArrowRight to plug the highlighted folder name into the search field,
- continue typing to further narrow down my search or to prepare a full name/path for a new folder to create.
Alternative Solutions
Without this “autosuggest” ability I either have to scroll through a long list of folder names or correctly type in the whole folder path. This is especially frustrating if I am trying to create a new subfolder.
Context
Thunderbird has a plugin “Manage Emails/ Nostalgy++” that provides similar functionality (“Save message to folder” in their vocabulary).