Marker line changes

- When a user changes to a new tab or to a different app, the marker is
reset.
- When changing to a tab that has its marker line reset and no new
messages have been displayed, the marker line doesn't appear on new
messages (while the user is still on that tab).
- When the marker gets to the top, it is not reset to avoid random
behavior (marker being reset and the user switching to that tab shortly
afterwards would make a few messages appear below the marker, while the
user hasn't seen any of the messages above it either).