I am running a Mac OS 10.11, using Photoshop CS5.
I have not used my Photoshop in awhile. When I open it and create a simple square with the Rectangular Marquee Tool, and then fill the rectangle I get a drop shadow effect on the edges.
I do not know if the effect is from drop shadow, it just has that appearance. I have tried multiple times to turn off drop shadow, but to avail. After doing so I try making a rectangular shape and filling it again --- but the effect is still there.
You hit it out of the park. I had completely forgotten about the feather settings!