Apply correct flexbox styling to .pane > .item-views > *

Markdown preview was overflowing because the min-height of flexbox
items is automatically set to min-content. Setting it to 0 ensures that
the item doesn't expand beyond its flex size. Moving this styling to
atom.css ensures that people don't have to work too hard to fit
their views into panes.

src/packages/markdown-preview/stylesheets/markdown-preview.css

@@ -3,9 +3,8 @@
   font-size: 14px;
   line-height: 1.6;
   background-color: #fff;
-  overflow: auto;
+  overflow: scroll;
   padding: 20px;
-  -webkit-flex: 1;
 }
 
 .markdown-preview pre,
@@ -40,10 +39,7 @@
 
 /* this code below was copied from https://github.com/assets/stylesheets/primer/components/markdown.css */
 /* we really need to get primer in here somehow. */
-.markdown-preview {
-  font-size: 14px;
-  line-height: 1.6;
-  overflow: hidden; }
+.markdown-preview {}
   .markdown-preview > *:first-child {
     margin-top: 0 !important; }
   .markdown-preview > *:last-child {

static/atom.css

@@ -61,6 +61,11 @@ html, body {
   -webkit-flex-flow: column;
 }
 
+#panes .pane .item-views > * {
+  -webkit-flex: 1;
+  min-height: 0;
+}
+
 @font-face {
   font-family: 'Octicons Regular';
   src: url("octicons-regular-webfont.woff") format("woff");

static/editor.css

@@ -6,7 +6,6 @@
   z-index: 0;
   font-family: Inconsolata, Monaco, Courier;
   line-height: 1.3;
-  -webkit-flex: 1;
 }
 
 .editor.mini {