If this is happening in Preview mode (the three pane window with message preview at lower right), then the only way to hide the preview pane is to have resized it by pulling its bottom up so that the preview pane is no longer visible. I think that condition would be obvious and intuitive to fix so I may be misunderstanding the preview pane that you are talking about. The two others that come to mind are the lower pane in a message reader window and the attachment preview pane. Please clarify.
FWIW, STATE.PMJ is the file that holds window settings. It holds a lot of other information too so a discard/rebuild is not advised without understanding the ramifications of doing so.
If this is happening in Preview mode (the three pane window with message preview at lower right), then the only way to hide the preview pane is to have resized it by pulling its bottom up so that the preview pane is no longer visible. I think that condition would be obvious and intuitive to fix so I may be misunderstanding the preview pane that you are talking about. The two others that come to mind are the lower pane in a message reader window and the attachment preview pane. Please clarify.
FWIW, STATE.PMJ is the file that holds window settings. It holds a lot of other information too so a discard/rebuild is not advised without understanding the ramifications of doing so.