weird result in template_preview for custom nodes

I’m testing the custom node editor template in 2.76a

import bpy
from bpy.types import NodeTree, Node, NodeSocket

# Implementation of custom nodes from Python

# Derived from the NodeTree base type, similar to Menu, Operator, Panel, etc.
class MyCustomTree(NodeTree):
    # Description string
    '''A custom node tree type that will show up in the node editor header'''
    # Optional identifier string. If not explicitly defined, the python class name is used.
    bl_idname = 'CustomTreeType'
    # Label for nice name display
    bl_label = 'Custom Node Tree'
    # Icon identifier
    bl_icon = 'NODETREE'

# Custom socket type
class MyCustomSocket(NodeSocket):
    # Description string
    '''Custom node socket type'''
    # Optional identifier string. If not explicitly defined, the python class name is used.
    bl_idname = 'CustomSocketType'
    # Label for nice name display
    bl_label = 'Custom Node Socket'

    # Enum items list
    my_items = [
        ("DOWN", "Down", "Where your feet are"),
        ("UP", "Up", "Where your head should be"),
        ("LEFT", "Left", "Not right"),
        ("RIGHT", "Right", "Not left")

    myEnumProperty = bpy.props.EnumProperty(name="Direction", description="Just an example", items=my_items, default='UP')

    # Optional function for drawing the socket input value
    def draw(self, context, layout, node, text):
        if self.is_output or self.is_linked:
            layout.prop(self, "myEnumProperty", text=text)

    # Socket color
    def draw_color(self, context, node):
        return (1.0, 0.4, 0.216, 0.5)

# Mix-in class for all custom nodes in this tree type.
# Defines a poll function to enable instantiation.
class MyCustomTreeNode:
    def poll(cls, ntree):
        return ntree.bl_idname == 'CustomTreeType'

# Derived from the Node base type.
class MyCustomNode(Node, MyCustomTreeNode):
    # === Basics ===
    # Description string
    '''A custom node'''
    # Optional identifier string. If not explicitly defined, the python class name is used.
    bl_idname = 'CustomNodeType'
    # Label for nice name display
    bl_label = 'Custom Node'
    # Icon identifier
    bl_icon = 'SOUND'

    # === Custom Properties ===
    # These work just like custom properties in ID data blocks
    # Extensive information can be found under
    myStringProperty = bpy.props.StringProperty()
    myFloatProperty = bpy.props.FloatProperty(default=3.1415926)

    # === Optional Functions ===
    # Initialization function, called when a new node is created.
    # This is the most common place to create the sockets for a node, as shown below.
    # NOTE: this is not the same as the standard __init__ function in Python, which is
    #       a purely internal Python method and unknown to the node system!
    def init(self, context):'CustomSocketType', "Hello")'NodeSocketFloat', "World")'NodeSocketVector', "!")
'NodeSocketColor', "How")'NodeSocketColor', "are")'NodeSocketFloat', "you")

    # Copy function to initialize a copied node from an existing one.
    def copy(self, node):
        print("Copying from node ", node)

    # Free function to clean up on removal.
    def free(self):
        print("Removing node ", self, ", Goodbye!")

    # Additional buttons displayed on the node.
    def draw_buttons(self, context, layout):
        layout.label("Node settings")
        layout.prop(self, "myFloatProperty")
        layout.template_preview([0], show_buttons=False)        

    # Detail buttons in the sidebar.
    # If this function is not defined, the draw_buttons function is used instead
    def draw_buttons_ext(self, context, layout):
        layout.prop(self, "myFloatProperty")
        # myStringProperty button will only be visible in the sidebar
        layout.prop(self, "myStringProperty")

    # Optional: custom label
    # Explicit user label overrides this, but here we can define a label dynamically
    def draw_label(self):
        return "I am a custom node"

### Node Categories ###
# Node categories are a python system for automatically
# extending the Add menu, toolbar panels and search operator.
# For more examples see release/scripts/startup/

import nodeitems_utils
from nodeitems_utils import NodeCategory, NodeItem

# our own base class with an appropriate poll function,
# so the categories only show up in our own tree type
class MyNodeCategory(NodeCategory):
    def poll(cls, context):
        return context.space_data.tree_type == 'CustomTreeType'

# all categories in a list
node_categories = [
    # identifier, label, items list
    MyNodeCategory("SOMENODES", "Some Nodes", items=[
        # our basic node
    MyNodeCategory("OTHERNODES", "Other Nodes", items=[
        # the node item can have additional settings,
        # which are applied to new nodes
        # NB: settings values are stored as string expressions,
        # for this reason they should be converted to strings using repr()
        NodeItem("CustomNodeType", label="Node A", settings={
            "myStringProperty": repr("Lorem ipsum dolor sit amet"),
            "myFloatProperty": repr(1.0),
        NodeItem("CustomNodeType", label="Node B", settings={
            "myStringProperty": repr("consectetur adipisicing elit"),
            "myFloatProperty": repr(2.0),

def register():

    nodeitems_utils.register_node_categories("CUSTOM_NODES", node_categories)

def unregister():


if __name__ == "__main__":

I have only added this line

layout.template_preview([0], show_buttons=False)        

I have created a texture for testing.

The problem is that preview sometimes is empty until the node get the focus.

Is this a bug?

I have seen that the texture panel (on the object properties) is not working when the nodes are active.

If I change the size of one preview in one node (make higher or lower), the preview size of the other nodes change too.

I have opened bug T46733 and the answer was that this is designed in this way and it’s pending in TODO list