Steps: 
- start GIMP with --gegl-threads=4
- open-as-layers 2 or more opaque images (or create an image with 2+ opaque layers)
- use the Move Tool to move the layers around and/or downscale the layers using Layer > Scale Layer

Result:
 the canvas area outside the uppermost layer is not composited correctly, mostly black. 

Further:
- the black areas in the mis-composition are often aligned with GEGL tiles;
- this problem does not happen using the transform tools;
- if you add alpha channel to *all* the layers the image is then composited correctly;
- this is not just a display-shell problem - duplicating the image or creating a new view of the same image shows much the same mis-composition, and the same thing happens when the image is created/manipulated programmatically from Python or Script-Fu console (without being displayed).

This seems to be a general problem with the way GIMP+GEGL deal with non-aligned, non-alpha layers. (The transform tools automatically add alpha channel and so do not elicit the problem.)

Note: just noticed this also happens on a build of Otto's gimp-edge from commit 9cc634e which was dated 2015-11-21, so this is not due to a change made in the last few weeks.