Avoid creating a new layer by mistake by drag&drop clip on a layers separator
Currently when dragging a clip between layers, the hovered separator is highlighted immediately, and dropping results in the creation of a new layer. It's very easy to create a new layer accidentally because the clip still appears on an existing layer.
We could highlight the separator only after hovering the separator without moving the mouse for a few seconds, to make sure this is intentional. See the
Timeline.dragEnd method in pitivi/timeline/timeline.py which contains the relevant logic.
(Next, we could display the clip widget floating to make sure the user does not have any unwelcome surprise.)