On the HiDPI display, everything looks nice. On the lower DPI one, elements are doubled in size. Other GNOME applications don't suffer the same problem.

This is on a fresh installation of Ubuntu 15.10 on my ThinkPad W451 (2880x1620) with an external DELL display (1920x1080).

Is Wayland involved here? My understanding was that GTK didn't support different scale factors on different X11 monitors.

Gdk/WindowScalingFactor, Gdk/UnscaledDPI, and Xft/DPI are XSettings, and so come from the Screen. X11 usually has one Screen for multiple monitors, and so I don't know how different values would be found. https://git.gnome.org/browse/gtk+/tree/gdk/x11/gdksettings.c?id=57d034ce9f16c883f2f91dd2cc3b3070a8e665e2#n73

No, Ubuntu still uses X11 AFAIK and doesn't even plan to use Wayland.

Some interfaces will be changed by bug 890156 to support multi monitor DPI setting support.

(In reply to Marco Castelluccio [:marco] (PTO until August 24/25) from comment #0) > Other GNOME applications don't suffer the same problem. I have a report that gtk3-demo suffers the same problem but Unity applications don't. Are you sure these were GNOME applications? I wonder whether Ubuntu has a modified libgtk-3 to listen to changes in GSettings com.ubuntu.user-interface scale-factor or dconf /com/ubuntu/user-interface/scale-factor. man 7 dconf says "The dconf API is not particularly friendly, and is not guaranteed to be stable. Because of this and the lack of portability, you almost certainly want to use some sort of wrapper API around it. The wrapper API used by GTK+ and GNOME applications is GSettings[1], which is included as part of GLib. GSettings has backends for Windows (using the registry) and Mac OS (using property lists) as well as its dconf backend and is the proper API to use for graphical applications." but last I looked GSettings was only really appropriate for application settings, because there wasn't a good way to test for the presence of settings. I wonder whether there is a better way to get system settings. Perhaps there is a system daemon that reads the GSettings and notifies dbus listeners. I haven't yet found documentation on these settings. https://bugs.eclipse.org/bugs/show_bug.cgi?id=489771 http://askubuntu.com/questions/510457/how-do-i-get-the-value-of-display-scale-for-menu-and-title-bars-from-the-c

Did you ever find a solution for this bug? I have the same problem in Gnome in Ubuntu 17.10.