Red line across screen

We can't diagnose that kind of issue remotely, sorry.

I think you'd better make an appointment at your most convenient Apple Store or Authorized Service Provider and have your iP8 inspected. See this list: https://locate.apple.com/findlocations

Regards

Giulio

Hello,

See this:

Get help with the display on your iPhone, iPad, or iPod touch

Good luck.

You're welcome.