mail - cannot open message viewer
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