Guard against PaneAxis as first child

Pane::findLeftmostSibling expected to return a Pane, not a PaneAxis

spec/workspace-spec.coffee

@@ -127,6 +127,22 @@ describe "Workspace", ->
             expect(pane1.items).toEqual [editor]
             expect(pane2.items).toEqual []
 
+      describe "when a pane axis is to the left of the current pane", ->
+        it "opens the new item in the current pane", ->
+          editor = null
+          pane1 = workspace.activePane
+          pane2 = pane1.splitLeft()
+          pane3 = pane2.splitDown()
+          pane1.activate()
+          expect(workspace.activePane).toBe pane1
+
+          waitsForPromise ->
+            workspace.open('a', split: 'left').then (o) -> editor = o
+
+          runs ->
+            expect(workspace.activePane).toBe pane1
+            expect(pane1.items).toEqual [editor]
+
       describe "when the 'split' option is 'right'", ->
         it "opens the editor in the rightmost pane of the current pane axis", ->
           editor = null

src/pane.coffee

@@ -339,15 +339,19 @@ class Pane extends Model
     newPane.activate()
     newPane
 
-  # If the parent is a horizontal axis, returns its first child;
-  # otherwise this pane.
+  # If the parent is a horizontal axis, returns its first child if it is a pane;
+  # otherwise returns this pane.
   findLeftmostSibling: ->
     if @parent.orientation is 'horizontal'
-      @parent.children[0]
+      [leftmostSibling] = @parent.children
+      if leftmostSibling instanceof PaneAxis
+        this
+      else
+        leftmostSibling
     else
       this
 
-  # If the parent is a horizontal axis, returns its last child;
+  # If the parent is a horizontal axis, returns its last child if it is a pane;
   # otherwise returns a new pane created by splitting this pane rightward.
   findOrCreateRightmostSibling: ->
     if @parent.orientation is 'horizontal'