docs: gtk/running: Document the GTK_CSD env var

It was not documented before.

https://bugzilla.gnome.org/show_bug.cgi?id=778791#c1

diff --git a/docs/reference/gtk/running.sgml b/docs/reference/gtk/running.sgml
index 1d8048dff494dceabc8bd4d44103fac00e921af8..0794064e71959ff458bfcafe325cd81b7a844ccf 100644 (file)
--- a/docs/reference/gtk/running.sgml
+++ b/docs/reference/gtk/running.sgml
@@ -514,6 +514,21 @@ nevertheless.
   </para>
 </formalpara>
 
+<formalpara>
+  <title><envar>GTK_CSD</envar></title>
+
+  <para>
+    The default value of this environment variable is 1. If changed to 0, this
+    disables the default use of client-side decorations on GTK+ windows, thus
+    making the window manager responsible for drawing the decorations of
+    windows that do not have a custom titlebar widget.
+  </para>
+  <para>
+    CSD is always used for windows with a custom titlebar widget set, as the WM
+    should not draw another titlebar or other decorations around the custom one.
+  </para>
+</formalpara>
+
 <formalpara>
   <title><envar>XDG_DATA_HOME</envar>, <envar>XDG_DATA_DIRS</envar></title>