Fix perf regression caused by flex-flow: column on pane item views

For some reason, having flex-flow column on the pane item views
container was forcing the entire view to be repainted on cursor
movement and line edits. Allowing the container to flow row-wise and
setting the min-height to 0 instead of min-content-height achieves
the same layout effect without the huge repaint cost. Flexbox is a
fragile bitch.

static/root-view.less

@@ -57,7 +57,7 @@ html, body {
   .pane .item-views {
     -webkit-flex: 1;
     display: -webkit-flex;
-    -webkit-flex-flow: column;
+    min-height: 0;
   }
 
   .pane .item-views > * {