I've just installed Windows 10, and when I set my display to 3840X2160 using display port connection, it doesn't fill the screen (with left and right black bar). But if I set to 4096X2160, it would fill the screen, but the aspect ratio is wrong, since my native resolution is 3840X2160.
My display is a 4K Toshiba TV, set to pc mode, native resolution is 3840X2160. My GPU is R7 370.
On Windows 7, using the same screen and GPU, I had no problem displaying 3840X2160 and it would fill the entire screen, so maybe it's a driver issue?
Also, if I set the resolution to 1920X1080 using my display port connection, I have the same problem not filling the screen. If I switch to hdmi connection using 1080p resolution, it would fill the screen, but I'm limited to max resolution 1920X1080 and can not use 4K resolution. So the problem happens only when using display port connection to have 4K display under windows 10.
If there's a way to fix the problem?
I found what was the problem, Windows 10 set the wrong refresh rate 3840X2160 @ 60htz instead of 30htz. Setting it to my TV native resolution 3840X2160 @ 30htz would fill the screen and the display/text looks 10 times better...