Absolute position pane resize-handle

- Keeps most themes unaffected
- More consistent with the tree-view resizer

static/panes.less

@@ -4,30 +4,39 @@
 // settings-view, the archive-view, the image-view. Etc. Basically a non-
 // editor resource with a tab.
 atom-pane-container {
+  position: relative;
   display: -webkit-flex;
   -webkit-flex: 1;
 
-  atom-pane-axis.vertical {
+  atom-pane-axis {
     display: -webkit-flex;
     -webkit-flex: 1;
+
+    & > atom-pane-resize-handle {
+      position: absolute;
+      z-index: 3;
+    }
+  }
+
+  atom-pane-axis.vertical {
     -webkit-flex-direction: column;
 
     & > atom-pane-resize-handle {
+      width: 100%;
       height: 8px;
-      z-index: 3;
+      margin-top: -4px;
       cursor: ns-resize;
       border-bottom: none;
     }
   }
 
   atom-pane-axis.horizontal {
-    display: -webkit-flex;
-    -webkit-flex: 1;
     -webkit-flex-direction: row;
 
     & > atom-pane-resize-handle {
       width: 8px;
-      z-index: 3;
+      height: 100%;
+      margin-left: -4px;
       cursor: ew-resize;
       border-right: none;
     }