diff --git a/spec/pane-resize-handle-element-spec.coffee b/spec/pane-resize-handle-element-spec.coffee
new file mode 100644
index 000000000..441be8028
--- /dev/null
+++ b/spec/pane-resize-handle-element-spec.coffee
@@ -0,0 +1,98 @@
+PaneContainer = require '../src/pane-container'
+PaneAxisElement = require '../src/pane-axis-element'
+PaneAxis = require '../src/pane-axis.coffee'
+
+describe "PaneResizeHandleElement", ->
+  describe "resize", ->
+    [container, containerElement, resizeElementMove, getElementWidth] = []
+
+    beforeEach ->
+      container = new PaneContainer
+      containerElement = atom.views.getView(container);
+      containerElement.style.minHeight = 100
+      document.querySelector('#jasmine-content').appendChild(containerElement)
+
+      resizeElementMove = (resizeElement, clientX, clientY) ->
+        mouseDownEvent = new MouseEvent 'mousedown',
+          { view: window, bubbles: true, button: 0 }
+        resizeElement.dispatchEvent(mouseDownEvent)
+
+        mouseMoveEvent = new MouseEvent 'mousemove',
+          { view: window, bubbles: true, clientX: clientX, clientY: clientY}
+        resizeElement.dispatchEvent(mouseMoveEvent)
+
+        mouseUpEvent = new MouseEvent 'mouseup',
+          {view: window, bubbles: true, button: 0}
+        resizeElement.dispatchEvent(mouseUpEvent)
+
+      getElementWidth = (element) ->
+        element.getBoundingClientRect().width
+
+    it "drag the pane resize handle element, then the panes around it will resize", ->
+      activePane = container.getActivePane()
+      rightPane = activePane.splitRight()
+
+      resizeElement = containerElement.querySelector('atom-pane-resize-handle')
+      expect(resizeElement).toBeTruthy()
+
+      leftWidth = resizeElement.previousSibling.getBoundingClientRect().width
+      resizeElementMove(resizeElement, leftWidth/2, 0)
+      expect(activePane.getFlexScale()).toBeCloseTo(0.5, 0.1)
+      expect(rightPane.getFlexScale()).toBeCloseTo(1.5, 0.1)
+
+      downPane = activePane.splitDown()
+      # after split down, the horizontal panes retain
+      expect(rightPane.getFlexScale()).toBeCloseTo(1.5, 0.1)
+
+    it "drag the resize element, the size of other panes in the same direction will not change", ->
+      leftPane = container.getActivePane()
+      middlePane = leftPane.splitRight()
+      rightPane = middlePane.splitRight()
+
+      resizeElements = containerElement.querySelectorAll('atom-pane-resize-handle')
+      paneElements = containerElement.querySelectorAll('atom-pane')
+      expect(resizeElements.length).toBe(2)
+
+      expectPaneScale = (leftScale, middleScale, rightScale) ->
+        expect(leftPane.getFlexScale()).toBeCloseTo(leftScale, 0.1)
+        expect(middlePane.getFlexScale()).toBeCloseTo(middleScale, 0.1)
+        expect(rightPane.getFlexScale()).toBeCloseTo(rightScale, 0.1)
+
+      resizeElementMove(resizeElements[0], getElementWidth(paneElements[0]) / 2)
+      expectPaneScale(0.5, 1.5, 1)
+
+      clientX = getElementWidth(paneElements[0]) + getElementWidth(paneElements[1]) / 2
+      resizeElementMove(resizeElements[1], clientX)
+      expectPaneScale(0.5, 0.75, 1.75)
+
+    it "drag the horizontal element, the size of other vertical pane will not change", ->
+      upPane = container.getActivePane()
+      downPane = upPane.splitDown()
+
+      [upRightPane, upLeftPane] = [upPane.splitRight(), upPane]
+      upPane = upLeftPane.getParent()
+
+      [downRightPane, downLeftPane] = [downPane.splitRight(), downPane]
+      downPane = downLeftPane.getParent()
+
+      horizontalResizeElements = containerElement.querySelectorAll('atom-pane-resize-handle.horizontal')
+      verticalResizeElement = containerElement.querySelector('atom-pane-resize-handle.vertical')
+      expect(horizontalResizeElements.length).toBe(2)
+
+      expectCloseTo = (element, scale) ->
+        expect(element.getFlexScale()).toBeCloseTo(scale, 0.1)
+
+      expectPaneScale = (up, down, upLeft, upRight, downLeft, downRight) ->
+        paneScales = [
+          [upPane, up], [downPane, down], [upLeftPane, upLeft],
+          [upRightPane, upRight], [downLeftPane, downLeft], [downRightPane, downRight]
+        ]
+        expectCloseTo(e[0], e[1]) for e in paneScales
+
+      newWidth = getElementWidth(horizontalResizeElements[0].previousSibling) / 2
+      resizeElementMove(horizontalResizeElements[0], newWidth)
+      expectPaneScale(1, 1, 0.5, 1.5, 1, 1)
+
+      newWidth = getElementWidth(horizontalResizeElements[1].previousSibling) / 2
+      resizeElementMove(horizontalResizeElements[1], newWidth)
+      expectPaneScale(1, 1, 0.5, 1.5, 0.5, 1.5)
diff --git a/src/pane-resize-handle-element.coffee b/src/pane-resize-handle-element.coffee
index 74b2a1989..80daecdd8 100644
--- a/src/pane-resize-handle-element.coffee
+++ b/src/pane-resize-handle-element.coffee
@@ -10,6 +10,7 @@ class PaneResizeHandleElement extends HTMLElement
 
   attachedCallback: ->
     @isHorizontal = @parentElement.classList.contains("horizontal")
+    @classList.add if @isHorizontal then 'horizontal' else 'vertical'
 
   resizeToFitContent: ->
     # clear flex-grow css style of both pane