Probably an oversight on my part but I have yellow handles at the beginning and end of most of my slides however on a couple there are only one yellow handle at the beginning. You cannot add or remove or do anything with the yellow bit itself. Basically the transition is set to a “dissolve” which is the same as the rest but it “cuts” to the next slide.
I dragged clips on all at the same time so wondering what happened.
The transition your setup for a certain slide is the one from the previous slide to the current one. For a dissolve the yellow box on the left hand side of the visibility and animation boxes describes the duration of the transition for this current slide (e.g. a dissolve needs the complete transition duration, for a fade it is just half the time). A yellow box on the right side shows the transition duration of the next slide depending on the settings in that very next slide. If there is no yellow box at the end of the current slide you properly have setup the next slide to make a cut transition.
Thanks Achim for the explanation - sorry about the delay but had to move on to another project.