User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:63.0) Gecko/20100101 Firefox/63.0

Steps to reproduce:

1. Open `about:config`, find `layout.css.devPixelsPerPx` and change the value from default (-1.0) to 1.2.
2. Open a view-source view with long lines: `view-source:https://www.youtube.com/user/10asti10`
3. hit Ctrl+f, search for rss
4. using the LMB, try selecting some text, then copy and paste it somewhere

Actual results:

After step 3, the highlighted text on the page is not ”rss”. The highlight doesn’t even adhere to letter widths (screenshot 1).

After step 4, the pasted text is not even close to what you’ve selected to copy.

Expected results:

For the search to hightlight an occurence of ”rss”, and for the pasted text to correspond to what I’ve selected from the view-source view.