[Bug 1798103] [NEW] Text highlight mismatch in view-source when layout.css.devPixelsPerPx is >= 1.2

[Jani Uusitalo]

Public bug reported:

== The issue ==
Search and text selection highlighting visuals get mismatched with the actual selection in view-source when the value of `layout.css.devPixelsPerPx` is 1.2 or larger.

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

== What happens ==
Screenshot 1: the highlighted text on the page is not ”rss”. The highlight doesn’t even adhere to letter widths.

Screenshot 2: the pasted text is not even close to what you’ve selected
to copy.

== What I expect to happen ==
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.

== Other info ==
* In my testing, a devPixelsPerPx value of 1.1 seems to not trigger this, whereas 1.2 and anything above does.
* In addition to 18.04 (whence the attached info is from) I’ve reproduced this in a VM running up-to-date 18.10.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: firefox 62.0.3+build1-0ubuntu0.18.04.1
ProcVersionSignature: Ubuntu 4.15.0-36.39-generic 4.15.18
Uname: Linux 4.15.0-36-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
BuildID: 20181002130007
CurrentDesktop: ubuntu:GNOME
Date: Tue Oct 16 16:59:28 2018
InstallationDate: Installed on 2016-10-13 (733 days ago)
InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fi_FI.UTF-8
 SHELL=/bin/bash
SourcePackage: firefox
UpgradeStatus: No upgrade log present (probably fresh install)

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug bionic

** Attachment added: "highlighted text on the page is not ”rss”"
   https://bugs.launchpad.net/bugs/1798103/+attachment/5201764/+files/screenshot-1.png

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https://bugs.launchpad.net/bugs/1798103

Title:
  Text highlight mismatch in view-source when layout.css.devPixelsPerPx
  is >= 1.2

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