I accidentally minimised (yellow button) a reply I was writing, and it zipped into the Dock. I have "minimise to app icon" enabled in Dock settings.
There's no way to get the window back that I can find. It doesn't show up in the Window menu, or right-clicking the Dock icon for Show All Windows, or cmd-backtick, or zoom, or anything else I can work out.
If I don't have "minimise to app icon" set in Dock prefs, the Compose window goes into the Dock in its own space and comes out again no trouble.
There should be a list of the open windows under Windows - I suspect this would automatically give you the right right-click Dock icon menu entries too.
Restarting Usenapp interestingly reset the main window view options as previously noted elsewhere. Maybe it's being overwritten by the compose window settings when quit with main + compose window open?
Should there be a Drafts folder? That's a bigger job.