(Version 1.7.0): After following instructions from Affinity Photo: Extract/Cut Out an Image and Selection with the Selection Brush in Affinity Photo, under Refine Selection I chose Output > New Layer with Mask.
OK, but now how do I re-select the selection (that I turned into a Mask) to edit the Mask further?
- Click Mask thumbnail. Then (via menu): Select > Selection from Layer (marching ants should now be visible); then Select > Edit Selection As Layer (or click Toggle Quick Mask in Toolbar). Quick Mask’s Red Overlay should now be visible.
- Command-click (NOT Control-click) the Mask Layer. Then Select > Edit Selection As Layer (or click Toggle Quick Mask). Quick Mask’s Red Overlay should now be visible.
Now choose the Selection Brush Tool and add/subtract.