Autoscroll correctly to the end after sending a message

On some systems when an encrypted message was written and the lock
icon was displayed, the textview did not scroll to the end.

Fixes #9443
3 jobs for master in 2 minutes and 27 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #13578
run-appdata

00:00:07

passed #13577
run-tests

00:02:10

 
  Build
passed #13579
run-build

00:00:09