Starting recently (but not aligned in my memory to any particular Software Update), when my MacBook Pro 13" Retina (late 2015) wakes from sleep with the external monitor connected (whether sleep was initiated by idle timeout or manually by pressing ⇧⌃⏻ (shift-control-power)), my Thunderbolt-connected display (an LG 34UC97-S 34” Curved Ultrawide 21:9 WQHD monitor, maximum resolution 3440×1440) comes up underscanned—fuzzy pixels with wide black bars on all sides, as so:
As you see, the Displays control panel shows it as "Default for display":
And, in fact, if I take a screen shot, the resolution of the image is the full 3440×1440.
If I switch the Resolution radio button to "Scaled", I see this, supporting that full resolution:
Holding down ⌥ (option) so that I get the "Detect displays" button and clicking it does not change anything. However, if I change the Scaled resolution to anything else (say, 2560×1080) and back to 3440×1440 or "Default for display", then the underscan goes away:
(I've included both photographs because from my web searching, people with similar issues are often told they're mistaking the monitor bezel for underscan bars. That's clearly not the case here.)
Again, this is a fairly new (perhaps a couple weeks old) issue—the monitor used to wake up with the correct resolution. When I disconnect and reconnect the monitor, sometimes it takes several tries to detect the display ("Detect displays" with ⌥ in the Displays panel doesn't work), and I believe that issue may have first cropped up around the same time.
I use the Thunderbolt port because I use peripherals connected to the display.
Best Answer
Try resetting your VRAM by restarting your macbook and pressing CMD+ALT+P+R when you hear the starting sound and hold until you hear the sound again. If you have a firmware password enabled you will have to turn this off in order to reset your VRAM.