Saving an .ora file and then importing it in GIMP 2.8 introduces a <mypaint background colour> edge to appear in GIMP.

1) In GIMP 2.8, create a new, solid black image, A4 at 300DPI (2480x3500 pixels). Save as .ora.

2) Open file in MyPaint. Immediately save it.

3) Re-open file in GIMP. You will notice the Mypaint background (white on my setup) shine through in the lower right.

I used to think this was due to me accidentally painting outside the image (and thus expanded the infinite canvas), but as shown it appears regardless of if I paint or not. It's easy to fix with the crop tool in GIMP, but it's nevertheless worth noting.
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Griatch