plotutils: Fix scene layout tracking in AnchorItem

Since 384f6f4 the AnchorItem does not properly update its layout
due to difference in pg.GraphicsObject/pg.GraphicsWidget
implementations.

Orange/widgets/visualize/utils/plotutils.py

@@ -11,7 +11,7 @@
  QFont, QPalette
 from AnyQt.QtWidgets import (
  QGraphicsLineItem, QGraphicsSceneMouseEvent, QPinchGesture,
- QGraphicsItemGroup, QWidget
+ QGraphicsItemGroup, QWidget, QGraphicsWidget
 )
 
 import pyqtgraph as pg
@@ -39,7 +39,8 @@ class AnchorItem(pg.GraphicsWidget):
  def __init__(self, parent=None, line=QLineF(), text="", **kwargs):
  super().__init__(parent, **kwargs)
  self._text = text
- self.setFlag(pg.GraphicsObject.ItemHasNoContents)
+ self.setFlag(QGraphicsWidget.ItemSendsScenePositionChanges)
+ self.setFlag(QGraphicsWidget.ItemHasNoContents)
 
  self._spine = QGraphicsLineItem(line, self)
  angle = line.angle()
@@ -57,6 +58,8 @@ def __init__(self, parent=None, line=QLineF(), text="", **kwargs):
  if parent is not None:
  self.setParentItem(parent)
 
+ self.__updateLayout()
+
  def get_xy(self):
  point = self._spine.line().p2()
  return point.x(), point.y()
@@ -131,6 +134,18 @@ def changeEvent(self, event):
  self._label.setColor(self.palette().color(QPalette.Text))
  super().changeEvent(event)
 
+ def itemChange(self, change, value):
+ if change in (
+ QGraphicsWidget.ItemParentHasChanged,
+ QGraphicsWidget.ItemSceneHasChanged,
+ # ItemScenePositionHasChanged seems to trigger for any scene
+ # transform change (even if the pos has not actually changed).
+ # Das ist gut.
+ QGraphicsWidget.ItemScenePositionHasChanged,
+ ):
+ self.__updateLayout()
+ return super().itemChange(change, value)
+
 
 class HelpEventDelegate(QObject):
  def __init__(self, delegate, parent=None):