Side-by-side and stacked views should indicate when the divider is dragged out of sight
When viewing a score with View documents side by side or stacked enabled it is possible to drag the divider to the edge of the view port where it disappears leaving a single view pane that is indistinguishable from the normal view. However, the relevant box in the view menu remains ticked. In this state, unticking and reticking the view side-by-side or stacked option has no visual effect. The divider can be dragged back into view but only if the user knows it is there.
I suggest that to avoid potential user confusion, a divider that is dragged to the edge of a view port should leave a handle visible to indicate that it is there.