Strange navbar always on bottom of the screen I have this problem for more than a month and I just can't find the problem that causes it. Can someone can help find the reason for it? Another problem with it is that you can't see it in the screenshots I took, what can I do, I feel helpless 
I have Nexus 6P
Judging from the edited-in camera shot of your screen, unfortunately you've fallen victim to the notorious screen burn-in problem of OLED displays. Since Nexus 6P is a 2-yr-old phone at this point, most of them probably already have it by now, albeit in different severity (mine has a very light shade of the navbar burnt in, too).

This is a hardware aging problem and can only be fixed by replacing the screen assembly. Apps that claim to "fix" the problem usually attempt to burn in a negative image of what's already burnt-in, which only worsens the color characteristics of the screen and makes it deteriorate even faster.