Strange border on shape and other layers

Question

I am working on a file. I have a background layer. I draw a shape layer, in this case a box, and when I do this layer has what almost looks like a drop shadow on two sides, ie the two edges have darker pixels than the shape color. I see the same sort of behavior on an image layer, in this case a photo, where the pixels on two edges, not the same edges, have different looking pixels. There are no effects on these layers

Answer 1

Try the following. Observe very closely and see if you can tell what's happening.
Create a new document at 400px × 400px; RGB, 8 bit, White background, square pixel aspect ratio. Resolution setting doesn't matter.
Tap your "D" key to return Photoshop's default FG & BG colors to black & white. Tap your "F" key to toggle screen mode to "Full Screen Mode with Menu Bar."
Make the documents rulers visible if they are not (CTRL + R). Right click on either of the rulers to drop the menu and make sure the unit of measurement is set to display values in pixels.
Zoom in to 1600% anywhere in the middle of your document. You'll be able to see tick marks on the rulers that designate each and every row and column of pixels.
THAT'S
the invisible, intrinsic "grid" I'm talking about, present in every Photoshop document. Want to see Photoshop's user configurable grid? Tap the
CTRL + ' shortcut a couple times. Leave your grid visible. (I left mine invisible for better clarity in the screenshots)
Select your rectangle Custom Shape Tool. In the Options Bar drop the Geometry Options menu on the right side of the row of shape icons.
UNcheck the box next to "Snap to Pixels." Then in the Option Bar's Styles drop-down menu, make sure that "No Style" is selected, as shown at the left.
In your document window click and drag out a small rectangle shape, without regard to where you begin and end. Notice the antialiasing that is introduced by Photoshop if your shape's diagonally opposite corners aren't started and ended in precise alignment with the ruler tick marks. Drag out guidelines as I have in this second image and you'll see what I mean.
Select your Move Tool (V). With the Shape Layer highlighted and active in the Layers Palette, tap your keyboard's arrow keys. You will see that the shape will ONLY move in 1 pixel increments. Pay attention to the antialiasing along the straight edges of the rectangle shape. Notice how the rows of pixels on either side of the vector line don't change in color/brightness value?
Select your Path Selection Tool (
black
arrow "A"). Click inside the rectangle shape to select it; you'll see all 4 corner anchor points display as tiny gray filled-in boxes. Tap your keyboard's arrow keys again. Now, notice how you can move the vector outline in increments of a fraction of a pixel. You can do this with vector-defined paths (which you have just selected). In the previous step, you were using your Move Tool; its overriding function is to move pixels. The vector path just follows along, in 1 pixel incremental movements. Also notice when you move the vector path in these fraction-of-a-pixel increments, the rows of pixels which Photoshop generates as antialiasing will change in color/brightness. This antialiasing is recalculated for each fractional movement so that the edge of the shape
appears
to be positioned mid-pixel. But, because we know that the pixel is an indivisible unit of measurement for a raster-based image, this is only done to create an illusion that a shape's edge sits at mid-pixel.
I know this reply is long and wordy (I could have gone into a deeper explanation, but I'm hoping this will suffice), but there's really no way to explain it quickly. Hopefully, this exercise has given you a better understanding of what you were seeing before you started this thread, and why it happens.
The takeaway lesson is this: If you want dead accurate and hard, clean horizontal and vertical edges, make sure that the vector paths which define the shape are aligned to Photoshop's intrinsic, invisible pixel grid.