Remove style attribute from textarea during ZenMode activation

Closes internal https://dev.gitlab.org/gitlab/gitlabhq/issues/2466

app/assets/javascripts/zen_mode.js.coffee

@@ -38,6 +38,8 @@ class @ZenMode
     @active_checkbox = $(checkbox)
     @active_checkbox.prop('checked', true)
     @active_zen_area = @active_checkbox.parent().find('textarea')
+    # Prevent a user-resized textarea from persisting to fullscreen
+    @active_zen_area.removeAttr('style')
     @active_zen_area.focus()
 
   exitZenMode: =>

spec/javascripts/zen_mode_spec.js.coffee

@@ -29,6 +29,11 @@ describe 'ZenMode', ->
       enterZen()
       expect(Mousetrap.pause).toHaveBeenCalled()
 
+    it 'removes textarea styling', ->
+      $('textarea').attr('style', 'height: 400px')
+      enterZen()
+      expect('textarea').not.toHaveAttr('style')
+
   describe 'in use', ->
     beforeEach ->
       enterZen()