Sharing CSS code between Gtk and the Shell

[Giovanni Campagna <scampa giovanni gmail com>]

Hello Gtk and hello Shell developers,

In https://bugzilla.gnome.org/show_bug.cgi?id=687881 I promised I'd
try to address one of the big performance problems in the current
gnome-shell, which is the St theming system.
Currently St does an awful lot of string matching again and again, as
neither the style classes and element names, nor the property names
are interned or preprocessed. We rely on libcroco to do basic
tokenization, but that's it, everything else happens every time a
StThemeNode (~ GtkStyleContext) is computed. We introduced some
caching at the beginning of the 3.7 cycle, but to me that's just
papering over a bad implementation.
So I started to rewrite it. I streamlined selector matching, added
quarking to classes, tried to normalize css values at parsing time and
finally built a map of known CSS properties to StThemeNode fields. But
I don't have a selector tree, I don't have GtkBitmask optimizations, I
had to reimplement GtkCssValue, etc.
The patch is not complete (it's missing background properties), but
I'm thinking: why should I write this again? Someone else did this in
Gtk, can't we just use that?

And this prompted me to this mail: is there a way for gnome-shell to
reuse Gtk CSS implementation?
Ideally, we would just use GtkStyleContext outside GtkWidget, and
parts of Gtk's docs claim it is supported. But I'm curious on what
assumptions are made from GtkWidgetPath, and if those assumptions can
be worked around or lifted. Additionally, there are a number of
accessors that are not exposed by GtkStyleContext or GtkThemingEngine,
but are available only to Gtk internals.
Consider that we can't use gtk_render_*, since we use Cogl for
painting, not cairo.
The next issue with that straightforward approach is that St's theming
is slightly more powerul than Gtk's. Looking at the gtk docs, I see
Gtk has no support for box-shadow, icon-shadow (st extension),
background-position, background-size, outline, text-decoration,
text-align, as well as width, min-width and max-width. If we were to
use Gtk directly, we would need to move our implementation into Gtk,
or work around that limitation.
Also, I saw that Gtk has recently deprecated extension CSS properties.
We use a number of those in gnome-shell, and it's not always possible
to replace them with standard CSS2, although I guess it'd be possible
to use gtk_style_context_save() / add_class() / restore() like Gtk
themes do.

The second option is to share just code, in some form, either as a
private shared library that would be provided by Gtk
(libgtk-internal?) and developed by both teams, or just as a copy-lib,
with a massive s/Gtk/St/. I'm not sure what would be the cost of this
last possibility, but I'm still convinced it would be lower than
rewriting from scratch, and maintaining two different CSS
implementations in GNOME.

Looking forward to your comments,

Giovanni