A few steps:
1. View > Hide Sidebar
2. Drag the split view divider between the message list and the conversation view all the way to the right edge of the window, collapsing the conversation view
3. Control-click the toolbar to customize it to your liking
4. Resize the window
If you move your mouse pointer up to the right edge of the window, from the left side, you should get a special one-way expand cursor that will allow you to reopen the collapsed viewing pane: