Making the event box invisible prevents painting a gray
background when placed over a different background (such
as a table panel that is given a different background by
its theme).
Restore the use of freeze & thaw on transparent canvases, but
ensure that the implementation underlying the GDK window won't
change while a freeze is in effect.
The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.
Support GUI scaling in much the same way as on Windows, where
the OS setting ("org.gnome.desktop.interface.scaling-factor"
times "...text-scaling-factor") determines the scale that is
used for both graphics and GUI sizing.
As I understand it, a complete solution requires porting to
Gtk3. With Gtk2, the graphical part of a widget doesn't scale.
Text and image labels should scale correctly, though.
