widget: fix widget insertion with or without canvas.before. closes #1021.

kivy/graphics/instructions.pxd

@@ -33,7 +33,7 @@ cdef class Instruction:
     cdef void rremove(self, InstructionGroup ig)
 
 cdef class InstructionGroup(Instruction):
-    cdef list children
+    cdef public list children
     cdef InstructionGroup compiled_children
     cdef GraphicsCompiler compiler
     cdef void build(self)

kivy/graphics/instructions.pyx

@@ -595,6 +595,23 @@ cdef class Canvas(CanvasBase):
                 self._after = c
             return self._after
 
+    property has_before:
+        '''Property to see if the canvas.before is already created
+
+        .. versionadded:: 1.6.1
+        '''
+        def __get__(self):
+            return self._before is not None
+
+    property has_after:
+        '''Property to see if the canvas.after is already created
+
+        .. versionadded:: 1.6.1
+        '''
+        def __get__(self):
+            return self._after is not None
+
+
     property opacity:
         '''Property for get/set the opacity value of the canvas.
 

kivy/uix/widget.py

@@ -280,7 +280,7 @@ class Widget(EventDispatcher):
             children = self.children
             if index >= len(children):
                 index = len(children)
-                next_index = -1
+                next_index = 0
             else:
                 next_child = children[index]
                 next_index = canvas.indexof(next_child.canvas)
@@ -290,6 +290,9 @@ class Widget(EventDispatcher):
                     next_index += 1
 
             children.insert(index, widget)
+            # we never want to insert widget _before_ canvas.before.
+            if next_index == 0:
+                next_index = 1
             canvas.insert(next_index, widget.canvas)
 
     def remove_widget(self, widget):