Photoshop selection tools not working correctly

Question

I've been using Photoshop for a while now and I've never found an error like this.

I needed to use the selection tool to cut out a part of an image and then paste it on a layer on top. However, whenever I do this there is a very obvious transparent line where I made the selection remaining.

The image is an example, made a circular selection around stomach I used the lasso tool, Ctrl-x then Ctrl-Shift-V to paste in place and then this is how it ends up.

I don't know what to do. Sorry if I posted this question in the wrong place, I'm new to Stack Exchange.

Thank you very much!

Answer 1

Yes, you can not actually do this. Simply, put this yet another variation of the conflation error. Typically this comes up with vector drawings but it is actually a bitmap drawing error.

Simple solution is not to cut but copy. There is no need to delete what is under. See what happens is transparency is not coverage so 50% opaque + 50% opaque does not become 100% opaque but rather 75%. Yet 50% of a pixel is 50% opaque.