GitLab

The "slide" edit mode should work like this:

The object is edited in slide mode. The Slide mode allows you to modify the position of a clip in a timeline without modifying its duration or its in-point, but will modify the duration of the previous clip and in-point of the following clip so does not modify the overall timeline duration.

http://lazka.github.io/pgi-docs/#GES-1.0/enums.html#GES.EditMode.EDIT_SLIDE