Use ‘atom-pane-container’ custom tag name for PaneContainerElement

src/pane-container-element.coffee

@@ -75,6 +75,4 @@ class PaneContainerElement extends HTMLElement
     top: {x: boundingBox.left, y: boundingBox.top}
     bottom: {x: boundingBox.left, y: boundingBox.bottom}
 
-module.exports = PaneContainerElement = document.registerElement 'atom-pane-container',
-  prototype: PaneContainerElement.prototype
-  extends: 'div'
+module.exports = PaneContainerElement = document.registerElement 'atom-pane-container', prototype: PaneContainerElement.prototype

src/pane-view.coffee

@@ -43,7 +43,7 @@ class PaneView extends View
     @subscriptions.add @model.onDidDestroy(@onPaneDestroyed)
 
   afterAttach: ->
-    @container ?= @closest('.panes').view()
+    @container ?= @closest('atom-pane-container').view()
     @trigger('pane:attached', [this]) unless @attached
     @attached = true
 
@@ -169,7 +169,7 @@ class PaneView extends View
 
   splitDown: (items...) -> @model.getView(@model.splitDown({items})).__spacePenView
 
-  getContainer: -> @closest('.panes').view()
+  getContainer: -> @closest('atom-pane-container').view()
 
   focus: ->
     @element.focus()

src/workspace-view.coffee

@@ -66,7 +66,7 @@ class WorkspaceView extends View
   setModel: (@model) ->
     @horizontal = @find('.horizontal')
     @vertical = @find('.vertical')
-    @panes = @find('.panes').view()
+    @panes = @find('atom-pane-container').view()
     @subscribe @model.onDidOpen => @trigger 'uri-opened'
 
   beforeRemove: ->

static/panes.less

@@ -3,7 +3,7 @@
 // Pane-items are things that go inside a pane. Like the UI-Demo, the
 // settings-view, the archive-view, the image-view. Etc. Basically a non-
 // editor resource with a tab.
-.panes {
+atom-pane-container {
   display: -webkit-flex;
   -webkit-flex: 1;