I just installed today. I turned on the horizontal reading pane but nothing happens. The single and the vertical panes work fine.
Thanks
To clarify how Mailspring’s layout modes work: the three options in Preferences > Appearance are “List” (no reading pane, open threads in a full view), “Split” (vertical split — list on left, message on right), and “Split Vertical” (horizontal split — list on top, message below). If you selected the horizontal split option and it doesn’t appear to be working, make sure to click “Apply Layout” after selecting it, and then restart Mailspring. If it still doesn’t work, let us know your OS and Mailspring version and we can investigate further.