I just switched to Windows 8.1 and I noticed, that when I bring the cursor in the bottom left corner on the start screen and hover to show the start button then grey lines show in the upper left corner.
My idea is that it's a border from the recent apps menu but it shouldn't show.
Now when I started to make this question I tried it and it seems like I was right, I opened some modern ui apps and there are more lines.
For some reason you can't see the cursor but on the first two images it's just on the start button.
How could I fix this?? (I don't think you can turn off the recent apps menu, you can turn off hot corners / charms but that's different.)