It was already possible to mark selected messages as read, but it required a few clicks. Looks like many people prefer single-click action for this particular case. Probably because other mail clients have it or maybe it’s just a very common action.
So, I implemented it today as a core feature. You can find “Mark all as read” in the folder list options menu (below Compact and Empty options).
I wouldn’t be myself if I didn’t try to improve a requested feature even as obvious/simple as this one. So, if there are more folders with unread messages, you’ll be asked to choose action scope. It means you can now mark (with one click) all messages in a) all subscribed folders or b) only in current folder or c) current and its subfolders.
So, actually it’s up to three clicks (two if only the current folder has unread messages) for the case “mark all as read in current folder”, but when you have more such folders it will be faster. I hope it will work for everyone.