width 100% for editor on mobile; no preview

also  remove box-sizing mixin, use native CSS3

app/assets/stylesheets/common/base/user.scss

@@ -44,7 +44,7 @@
     .btn {
       width: 100%;
       margin-bottom: 5px;
-      @include box-sizing(border-box);
+      box-sizing: border-box;
     }
   }
   h2 {

app/assets/stylesheets/common/foundation/mixins.scss

@@ -33,14 +33,6 @@
 // CSS3 properties
 // --------------------------------------------------
 
-// Box sizing
-
-@mixin box-sizing($sizing) {
-  -webkit-box-sizing: $sizing;
-     -moz-box-sizing: $sizing;
-          box-sizing: $sizing;
-}
-
 // Border radius
 
 @mixin border-radius-all($radius) {

app/assets/stylesheets/desktop/compose.scss

@@ -367,7 +367,7 @@
     }
 
     #wmd-input, #wmd-preview-scroller, #wmd-preview {
-      @include box-sizing(border-box);
+      box-sizing: border-box;
       width: 100%;
       height: 100%;
       min-height: 100%;
@@ -397,7 +397,7 @@
     }
     .textarea-wrapper, .preview-wrapper {
       position: relative;
-      @include box-sizing(border-box);
+      box-sizing: border-box;
       height: 100%;
       min-height: 100%;
       margin: 0;

app/assets/stylesheets/mobile/compose.scss

@@ -238,7 +238,6 @@ display: none;
 
 
     #wmd-input {
-      @include box-sizing(border-box);
       width: 100%;
       height: 100%;
       min-height: 100%;
@@ -255,12 +254,11 @@ display: none;
     }
     .textarea-wrapper {
       position: relative;
-      @include box-sizing(border-box);
+      box-sizing: border-box;
       height: 100%;
       min-height: 100%;
       margin: 0;
       padding: 0;
-      width: 50%;
     }
     .textarea-wrapper {
       padding-right: 5px;