GtkGroupBox and layout ideas for Gtk3

Recently in my copious spare time (i.e. none, being on paternity leave)
I've been thinking about doing a Gtk+ container for Gtk3 based on the
java GroupLayout layout mechanism. This is a layout system explicitly
designed such that it is useful for an intuitive GUI editor (and its not
very useful for manual coding). This is the basis for the netbeans form
editor (Matisse). For a quick demo (of an ancient version, but you get
the point) see:

The general approach of GroupLayout is that you separate allocation in
horizontal and vertical orientations and for each direction you create a
recursive set of groups, each one arranging coordinates sequentially or
in parallel. See e.g:

In Gtk+ this would be done as a GtkGroupBox container with two GtkGroup
hierarchies (one per direction). With the setup in Gtk3 this should be
quite doable. However, there are a few things missing for a perfect

First of all, what happened to Bug 628902 "Add expand flags to
GtkWidget". It seems to have stalled. This would be very useful.

Secondly, the matisse layouts heavily feature baseline-aligned layouts,
which I think would help gtk+ a lot to reach the next level in gui
neatness. The problem with this is that its not enough to just add a
baseline property to the widget, because baseline handling must be part
of the size negotiation. For example, a baseline aligned hbox with to
labels, containing "Y" and "y", clearly would be taller than the maximum
height of the two labels. So, we need to change the
gtk_widget_get_preferred... APIs to include the baseline. Fortunately
these are not set in stone yet, so we have time before Gtk3 is released.

Third, rather than hardcoding spacing in containers, matisse is based on
adding spacers (gaps) to the layouts. These gaps are either of custom
size rigid (basically what we get in gtk now) or flexible, but mostly
they are of standard sizes, that are tied to the theme. For instance, it
has standard sizes for distance to container edge, distance between
related components, distance between unrelated components and distance
for indentation. 

In fact, not only do these distances depend on the theme, but their
exact values actually depend on the widgets in questions, such that e.g.
the distance between two related buttons could be different than between
a label and a related button.

I'm not sure the per-widget-type distances are all that useful, but at
the very least having the other standard distances available and themed
would both make Gtk+ code less riddled with constants, and allow themes
that have different amounts of whitespace.

As for manual layouting it seems all the java people swear by

 Alexander Larsson                                            Red Hat, Inc 
       alexl redhat com            alexander larsson gmail com 
He's a genetically engineered soccer-playing waffle chef looking for a cure to 
the poison coursing through his veins. She's an enchanted hip-hop traffic cop 
who can talk to animals. They fight crime! 

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