> If you rotate by exactly 90 degrees, this is always done with
> INTERPOLATION_NONE, no matter what you select in the tool options.
Perhaps this is the culprit? An offset seems unavoidable if the
transformation is performed without interpolation. So perhaps all we
need to do is to remove this optimization (which is supposed to speed up
rotations by multiple of 90 degrees)?
Peter, can you try to remove that code in
app/core/gimp-transform-region.c (line 208 and 209) and check if that
fixes your test case?
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