Mailboxes and Folders display panel disappears

Long time user of iCloud mail in both Apple and Windows environments. I'm accustomed to a three-panel display: Mailboxes and Folders on the left, headers in the middle, and selected email content on the right. It's been that way for ages.

I logged in one day through Windows and discovered I no longer have the Mailboxes/Folders panel but just the Headers and content panels. I didn't change any settings or remember fat-fingering anything. I've reluctantly become accustomed to this.

Logging in through Windows recently, I saw the three panels were back. Yay! That lasted for a few days but now the Mailboxes/Folders panel is gone again. Once more, I did nothing I recall that created this change.

What in the world is going on? I'm not losing any folders or content. I can work with it either way, but this is really annoying. Any ideas how to fix this so I can have the three-panel display?