Since updating to High Sierra, I can no longer expand (or even open) the message viewer pane. The bar used to resize the pane still appears, and the up-arrow indicator is shown when mouse is hovered over the bar, but the bar does not move.
Mac mini (Late 2014), OS X El Capitan (10.11.5), MacBook Pro 13, iPad Air,iPhone 6s
Position your cursor over that bar where it changes to an up-arrow indicator. Drag it up, way up, much more than you have been doing. There will be no visual confirmation of that action until you drag it more than halfway to the opposite edge of the window, or full screen if you are not using windowed mode. When you drag it a sufficient distance the message viewer pane will reappear.
mail - cannot open message viewer
