If you have a transition between two video clips, there is no (easy) way to change the length of the transition without adding/removing frames to/from one of the clips.
You can move the start of the transition to the right to make the transition shorter, but this will remove frames from the start of the second clip. If you move the end of the transition to the right to make it shorter, frames from the end of the first clip are removed.
To make a transition longer, you can move the start/end of the transition to the left/right, but this will add frames to the start/end of the second/first clip - frames you previously cut of from the start/end of the clip, frames you maybe don’t want to be visible. And it only works, if there are additional frames available at the start/end of the clip.
In some situations this behavior might be what you want, but it should be possible to change the length of a transition without adding/removing frames to/from the clips.
My suggestion: It should be possible to change the length of a transition by moving the second clip.
Currently, if you move the second clip to the right, you get a gap between the transition and the clip. That makes no sense, I can not think of any reason why anybody would need a gap between a transition and the following clip. And if you move the clip to the left, over the transition, it jumps back.
Instead, if you move the 2nd clip of a transition, the length of the transition should be changed accordingly. If you move the clip to the left, the transition should become longer, because the amount of overlapping between the two clips will be longer. If you move it to the right, the transition should become shorter. You may even remove the transition completely, if you move the clip far enough to the right.
This should only happen if you move the clip on the right side of a transition (the 2nd clip), but not if you move the first clip. Reason: If this would work with both clips and if you move a clip with transitions on both sides, you would change the transitions on both sides of the clip - and if you don’t see both transitions, because the clip is long, you may even not notice that you changed both transitions. But if only the transition on the left side of the clip is changed, all following transitions (on the right side of the moved clip) will stay unchanged. (You will get a gap after the clip, if you move it to the left, and you need a gap after the clip, before you can move it to the right.)