Hard to change an keyframable effect
With the colored vertical line added in v19 at the beginning and end of a track in the timeline it is very difficult to change a keyframable effect.
The green/red line is as wide as the rest of the red circle of the effect and makes it impossible to reach this circle with the mouse pointer.
At most you will catch the track.
See also https://forum.kde.org/viewtopic.php?f=270&t=164200
As it was in v18, the usage was better.
Example picture: https://de.share-your-photo.com/7b967c6c31 
AppImage 19.12.0 and 19.12.1
Edit by ME: added the picture and add line breaks.