Fixes #3736 - scrollPageHeader disappears when answering via email.

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

@@ -456,6 +456,8 @@ class App.TicketZoom extends App.Controller
     if !@renderDone
       @renderDone      = true
       @autosaveLast    = {}
+      @scrollHeaderPos = undefined
+
       elLocal = $(App.view('ticket_zoom')
         ticket:         @ticket
         nav:            @nav

spec/system/ticket/zoom_spec.rb

@@ -270,6 +270,25 @@ RSpec.describe 'Ticket zoom', type: :system do
         end
       end
     end
+
+    context 'scrollPageHeader disappears when answering via email #3736' do
+      let(:ticket) do
+        ticket = create(:ticket, group: Group.first)
+        create_list(:ticket_article, 15, ticket: ticket)
+        ticket
+      end
+
+      before do
+        visit "ticket/zoom/#{ticket.id}"
+      end
+
+      it 'does reset the scrollPageHeader on rerender of the ticket' do
+        select User.find_by(email: 'admin@example.com').fullname, from: 'Owner'
+        find('.js-textarea').send_keys('test 1234')
+        find('.js-submit').click
+        expect(page).to have_selector('div.scrollPageHeader .js-ticketTitleContainer')
+      end
+    end
   end
 
   describe 'delete article', authenticated_as: :authenticate do