1. Have at least 2 layers. Layer 1 and 2, 2 is at the top. 2. Try to click and drag and move Layer 1 above layer 2. There is a 1 pixel window/border where the cursor has to land and release to allow this move. This is a very precise requirement for a simple and common action. It seems to be an inefficient UX. Suggest: Increasing the target size to something like 10pixels or so to make it easier to 'land' on it.
I'm not even sure whether it's possible, but https://phabricator.kde.org/M11 is the mockup for the new layer docker, and I've added a note about increasing drop zones.
With the new layer docker you simply drop on the above layer and it appears above that layer.