UserAgent: SyntaxError: Unexpected token < in JSON at position 61126

Steps to reproduce the problem:
1. Open the attached test case in Chrome

What is the expected behavior?
No weird diagonal lines should appear.

What went wrong?
As the title says, some white diagonal lines appear in <pre> tag without any reasonable explanation. Modifying any part of the attached html file causes the diagonal lines to disappear. I'm also providing a screencast.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 63.0.3239.132  Channel: stable
OS Version: 6.3
Flash Version: /
 

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index.htm 482 bytes View Download

Deleted: 1.mp4 3.1 MB

	1.mp4 3.1 MB View Download