Merge changes from origin/7.1

30e9b7c Add a test for @mixin { @font-face {...} } (#11649)
0a2cf2c Prevent showing "Session Expired" notification if navigating away (#12298)
8efc385 Fixes NPE in TreeTable.setContainerDS(null) (#12928)
23815e6 Remove broken JSch check
c308d8a Fixes double scrollbar issue in browserframes (#11780) Fixed by adding font-size 0 to the base theme.

Change-Id: I64ae167e8e6b61205718998b80d534fb2cf0cb3d

diff --cc client/src/com/vaadin/client/ui/VUI.java
Simple merge

diff --cc client/src/com/vaadin/client/ui/ui/UIConnector.java
Simple merge