make scrollHeader fully dissapear

by using scroll offset of the scroll parent (previously the headers bottom border was still visible because it only scrolled up 62px instead its full height of 64px)

app/assets/javascripts/app/controllers/ticket_zoom.js.coffee

@@ -160,23 +160,22 @@ class App.TicketZoom extends App.Controller
     @main.unbind('scroll', @positionPageHeaderUpdate)
 
   positionPageHeaderUpdate: =>
-    pageHeader       = @scrollPageHeader.height()
+    headerHeight     = @scrollPageHeader.outerHeight()
     mainScrollHeigth = @main.prop('scrollHeight')
     mainHeigth       = @main.height()
 
     # if page header is possible to use, show page header
     top = 0
-    if mainScrollHeigth > mainHeigth + pageHeader
-      offset = @ticketZoom.offset()
-      if offset.top >= 0
-        top = offset.top
+    if mainScrollHeigth > mainHeigth + headerHeight
+      scroll = @main.scrollTop()
+      if scroll <= headerHeight
+        top = (scroll - headerHeight)
 
     # if page header is not possible to use - mainScrollHeigth to low - hide page header
     else
-      top = pageHeader
-
-    #console.log('TOP', top, @ticket.id, new Date)
-    @scrollPageHeader.css('transform', "translateY(-#{top}px)")
+      top = -headerHeight
+    
+    @scrollPageHeader.css('transform', "translateY(#{top}px)")
 
   render: (force) =>