Merge changes from origin/7.1

51a46a0 Recovering scroll position after regression problems. (#12727)
c464dcf Convert push test to TB3 so it has a chance to pass
2c6d010 Fixes test issues preventing it from passing
049b9d9 Remove out of order flush() (#12283)

Change-Id: I600294f41a4a1b7682f99bddb03042f84bee2702

diff --cc server/src/com/vaadin/server/VaadinServlet.java
Simple merge