Absolute placement for canvas gui dialogs (CSS like right/bottom margin)

Nasal/canvas/gui.nas

@@ -10,6 +10,9 @@ var Dialog = {
     m.setInt("size[0]", size_dlg[0]);
     m.setInt("size[1]", size_dlg[1]);
 
+    # arg = [child, listener_node, mode, is_child_event]
+    setlistener(m._node, func m._propCallback(arg[0], arg[2]), 0, 2);
+
     return m;
   },
   # Create the canvas to be used for this dialog
@@ -59,6 +62,51 @@ var Dialog = {
   raise: func()
   {
     me.setBool("raise-top", 1);
+  },
+# private:
+  _propCallback: func(child, mode)
+  {
+    # support for CSS like position: absolute; with right and/or bottom margin
+    if( child.getName() == "right" )
+      me._handlePositionAbsolute(child, mode, child.getName(), 0);
+    else if( child.getName() == "bottom" )
+      me._handlePositionAbsolute(child, mode, child.getName(), 1);
+  },
+  _handlePositionAbsolute: func(child, mode, name, index)
+  {
+    # mode
+    #   -1 child removed
+    #    0 value changed
+    #    1 child added
+
+    if( mode == 0 )
+      me._updatePos(index, name);
+    else if( mode == 1 )
+      me["_listener_" ~ name] = [
+        setlistener
+        (
+          "/sim/gui/canvas/size[" ~ index ~ "]",
+          func me._updatePos(index, name)
+        ),
+        setlistener
+        (
+          me._node.getNode("size[" ~ index ~ "]"),
+          func me._updatePos(index, name)
+        )
+      ];
+    else if( mode == -1 )
+      for(var i = 0; i < 2; i += 1)
+        removelistener(me["_listener_" ~ name][i]);
+  },
+  _updatePos: func(index, name)
+  {
+    me.setInt
+    (
+      index == 0 ? "x" : "y",
+      getprop("/sim/gui/canvas/size[" ~ index ~ "]")
+      - me.get(name)
+      - me.get("size[" ~ index ~ "]")
+    );
   }
 };