How to merge parts of an image aligned to their edges?

[felixcra]

Many thanks folks. The way that worked out best for me with this tipps is:

Create new layer containing upper part. -> Select lower part and add guides to selection. -> Move upper part/the new layer to the lower one and make them snap together using the created guides.

Make sure to enable:
-Snap to Canvas Edges
-Snap to Guides

Make sure to disable:
-Snap to Grid (Now idea why we need this one)

Hope this helps future readers