Monitor DPI settings are ignored after upgrade to Xenial

[Michael Marley]

I recently upgraded my Kubuntu x86_64 system with a HiDPI monitor (24" 4K) from Wily to Xenial. Before I did this, applications using all the toolkits (GTK2, GTK3, Qt4, and Qt5) properly obeyed the DPI value automatically detected from the monitor, causing them to automatically scale to a usable size. However, after upgrading to Xenial, GTK3 applications no longer have this behavior. Instead, they appear at the normal 96DPI size, causing them to be nearly unreadable due to the small size of their widgets.

I am currently using GTK3 3.18.7-1ubuntu2, but the issue has been occurring since I upgraded to Xenial months ago.

[Launchpad Janitor (janitor)]

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

[Flames_in_Paradise (ellisistfroh-deactivatedaccount)]

Ecxuses – just tested bug #1260760 (editing an Ubuntu (gtk+3.0) bug unsets the source)

[madbiologist (me-again)]

What is the output of

xdpyinfo | grep dimensions

and

xdpyinfo | grep resolution

[Michael Marley (mamarley)]

michael@mamarley-desktop:~$ DISPLAY=:0 xdpyinfo | grep dimensions
  dimensions: 7680x2160 pixels (1060x301 millimeters)
michael@mamarley-desktop:~$ DISPLAY=:0 xdpyinfo | grep resolution
  resolution: 184x182 dots per inch
michael@mamarley-desktop:~$

I should also mention that this bug now only happens for applications running as root, such as Synaptic. Applications run as the normal user display at the correct size now.