Sorry, I didn't realise it had been discussed before. I didn't realise at the time of writing, but the method is already implemented for panoramas. After stitching, if you select the crop tool while still in the panorama section you can "crop to opaque" before committing to the pano and being taken into the photo persona. It would be great if that could be brought into the develop and photo personas.
Welcome to the forum and sorry for the delay in replying.
AFAIK, it's not currently possible. Another user made the same request here: https://forum.affinity.serif.com/index.php?/topic/30860-ap-automatic-crop-for-the-straighten-tool/
New member here but I have been using the forums for information for a while, so than you to everybody who has unknowingly helped me so far! As you might have guessed though, I haven't found the answer to my latest question, which is:
Is it possible to have Photo automatically crop remaining dead space to the original aspect ratio (or maybe a predefined one from your presets), or with a single click after using the straighten tool, or manual rotation? I usually take landscape, or architectural type shots so doing it manually occasionally is no problem, but I'm finding it can be a little tedious when doing batches of images that all need adjusting by a few degrees. If it isn't possible, is it something that could be implemented? I think it would be incredibly useful!
I apologise if this has been asked before, i just couldn't find the answer.