[Gimp-user] Selection Lines Still Visible After Merge

[Rick Snedeker <rsned comcast net>]

There are 2 layers, a background and a 2nd.
I selected a portion of the 2nd layer and added a layer mask > Selection > Invert.
I then applied the layer mask which showed the intended image in the background layer below, but after a 
moment it refreshed and became obscured by the diagonal lines of my selection.
My next step was to merge the 2nd layer into the background layer, which momentarily looked correct but again 
became obscured by the diagonal selection lines.

Do I need to flatten the image to make the selection lines go away?

—Rick