There’s an easier way than copying + deleting, its mentioned on the other discussion about selection, it makes use of “destination in/out” layers. I’ve added a similar response there and documented my own workflow at github document selection workflow · Issue #1240 · mypaint/mypaint · GitHub so maybe it can be communicated better in the future.
What I’m doing is creating a new paint layer, switch it to “destination out” mode and paint on it with a thick opaque brush to “select” something, but since its a layer of its own its not permanent (yet) and I can review the selection by switching the mode to see exactly what I selected (“destination in”) or what I covered (normal mode) if its important for fine details.
Once I’ve drawn my “selection” this way I can use the layer visibility toggles together with “create new layer from visible” context menu to capture the “destination in” view into (which displays just the selection) into a new layer where it can be manipulated on its own. I can keep it on its own layer or merge it back after editing.
If I want to erase the selection from the original layer I use the “destination out” mode, toggle the visibility aprropriately and merge the selection into the original layer.
PS: might need to turn off background rendering while using “create layer from visible” or you may get black artifacts
