drscheme

I originally used selection of transparent pixels, but then some pixels at the edge didn't get selected. Turns out these were partly-transparent ones, created during the rotation algorithm. Fix for this is to use a 'grow' on the selection. Another, more reliable one, is to select only fully opaque pixels and invert the selection, which I now use.

For some reason they are not equivalent, strange results can occur sometimes
Always select the opaque areas and invert the selection for best (and consistent) results
PS Rasterising the layer first, before Inpainting, is crucial when certain other operations (such as cropping) has been done on the image

A *.afmacros file contains a collection of macros (usually more than one!) but a *.afmacro file always contains a single macro. You can only open *.afmacros files via the Library panel, and you can only open *.afmacro files via the Macros panel.
 
 

Dave shared this tip a couple of weeks ago:
https://forum.affinity.serif.com/index.php?/topic/41825-how-to-fill-in-corners-after-rotating/&do=findComment&comment=227167
Scroll in that string to where you see Dave re-posted with a correction to the steps to take.
BONUS: He even made a macro for the process and posted the link so you can download it.