Bring back position:fixed for tree view resizer

Without position fixed, the resizer breaks when the tree
view is scrolled.

src/packages/tree-view/src/tree-view.coffee

@@ -37,6 +37,7 @@ class TreeView extends ScrollView
     treeView.focusAfterAttach = state.hasFocus
     treeView.scrollTopAfterAttach = state.scrollTop
     treeView.width(state.width)
+    treeView.resizer.css(left: state.width)
     treeView.attach() if state.attached
     treeView
 

src/packages/tree-view/stylesheets/tree-view.css

@@ -16,6 +16,7 @@
   width: 10px;
   background: transparent;
   cursor: col-resize;
+  position: fixed;
 }
 
 .tree-view .entry {