system scaling factor ignored for page rendering

[Donald Pellegrino]

Mozilla Firefox has a layout.css.dpi preference with a default value of -1. This preference is documented as:

"Use the host system’s logical resolution or 96, whichever is greater, for interpreting dimensions specified in absolute units. (Default)" as per http://kb.mozillazine.org/Layout.css.dpi

On Linux, the value of -1 should cause Firefox to use the GTK scaling factor. Additional details are available in Mozilla Bug 712898 (https://bugzilla.mozilla.org/show_bug.cgi?id=712898).

Unity System Settings -> Displays includes a slider for "Scale for menu and title bars." When set to 1.12 instead of the default value of 1, the text on all GTK dialogs including body text, menus, and title bars are all scaled. In addition, the text in Terminal is scaled. In Firefox, the text in about:preferences is scaled consistent with the elements of other applications. However, the page rendering is not scaled. Page scaling seems to be unaffected by the system setting.

The expected behavior is that the scale value set in System Settings -> Displays should affect Firefox page rendering when the Firefox layout.css.dpi preference has its default value of -1.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: firefox 47.0+build3-0ubuntu0.16.04.1
ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
Uname: Linux 4.4.0-28-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: donald 1749 F.... pulseaudio
BuildID: 20160606113944
Channel: Unavailable
CurrentDesktop: Unity
Date: Thu Jul 7 19:26:57 2016
EcryptfsInUse: Yes
ExecutablePath: /usr/lib/firefox/firefox
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
IpRoute:
 default via 192.168.1.1 dev wlp2s0 proto static metric 600
 169.254.0.0/16 dev wlp2s0 scope link metric 1000
 192.168.1.0/24 dev wlp2s0 proto kernel scope link src 192.168.1.240 metric 600
LocalLibraries: /<email address hidden>/platform/Linux_x86_64-gcc3/components/lpxpcom_x86_64.so
Locales: extensions.sqlite corrupt or missing
PrefSources:
 prefs.js
 [Profile]/<email address hidden>/defaults/preferences/defaults.js
ProcEnviron:
 LANGUAGE=en_US
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=47.0/20160606113944 (In use)
RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 11/29/2015
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 1.05.06RS76
dmi.board.asset.tag: Tag 12345
dmi.board.name: Lemur
dmi.board.vendor: System76, Inc.
dmi.board.version: lemu6
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: System76, Inc.
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr1.05.06RS76:bd11/29/2015:svnSystem76,Inc.:pnLemur:pvrlemu6:rvnSystem76,Inc.:rnLemur:rvrlemu6:cvnSystem76,Inc.:ct10:cvrN/A:
dmi.product.name: Lemur
dmi.product.version: lemu6
dmi.sys.vendor: System76, Inc.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.