Sounds like you might have an issue with a mouse, perhaps a stuck button, but I rally can not see how that would work either. Moving over the divider pane will set the mouse to have a double left right arrow, clicking on the mouse when this occurs selects the pane for resizing and drags the pane to Zero width if you do not release the left button while dragging. This in effect set the pane to off. All I can think of is you are accidentally clicking and holding while in that state, perhaps trying to select and email and drag it somewhere else.