I'm not convinced that this is the ideal fix; there is something not write to me with having the signal/method conflict via multiple-inheritance.

Given that GraphicsItem.parentChanged() just called GraphicsItem._updateView() should we instead do something like

# in GraphicsObject.__init__
self.parentChanged.connect(self._updateView)

And get rid of GraphicsView.parentChanged method altogether.

Thoughts?

EDIT: On second thought, that would be a whole new behavior change; instead perhaps we should change the diff to call ._updateView() instead of .parentChanged() (and delete the .parentChanged() method altogether)

So the following would be the MWE. Python rules say that M is searched before Q.
The puzzling thing is: how does PySide6 6.2.2 manage to subvert the Python resolution machinery?

from PySide6 import QtCore

class Q(QtCore.QObject):
    signal = QtCore.Signal()

    def method(self):
        print('Q::method')

class M:
    def signal(self):
        print('M::signal')

    def method(self):
        print('M::method')

class C(M, Q):
    def __init__(self):
        Q.__init__(self)
        M.__init__(self)

c = C()
c.method()    # okay
c.signal()    # problem on PySide6 6.2.2

Documenting here that calling GraphicsItem.parentChanged(self) instead of self.parentChanged() prevents ScaleBar::parentChanged method from getting called.

I have a workaround that will successfully call ScaleBar.parentChanged.
Change in GraphicsObject.py:

           self.parentChanged()

to:

            import types
            if isinstance(self.parentChanged, types.MethodType):
                self.parentChanged()
            else:
                # workaround PySide6 6.2.2
                getattr(self.__class__, 'parentChanged')(self)

The else branch actually works for all bindings, but given that this appears to be a bug of PySide6 6.2.2 which may perhaps get fixed in the future, writing it this way documents the why.

Holding off on merging this until I can submit a bug-report with the pyside folks so I can reference the specific issue number; unfortunately I can't set 6.2.2 as the "affected version" for the submission process...hopefully I can do that tomorrow.

Created https://bugreports.qt.io/browse/PYSIDE-1730 with @pijyoi 's minimum reproducible example, added a comment to this issue, think this is ready for merging now 👍🏻

The following is a better test of any eventual fix in PySide6 and reflects pyqtgraph's usage. (ScaleBar)
In particular, on PySide6 6.2.2, print(d.signal) yields <PySide6.QtCore.SignalInstance object at ...>, whereas on PySide6 <= 6.2.1 and other bindings, it yields `<bound method D.signal of ...>'

It seems from the current discussion on https://bugreports.qt.io/browse/PYSIDE-1730 that a fixed version might still yield <PySide6.QtCore.SignalInstance object at ...>.

from PySide6 import QtCore

class Q(QtCore.QObject):
    signal = QtCore.Signal()

class M:
    def signal(self):
        # default implementation
        print("M::signal")

class C(M, Q):
    pass

class D(C):
    def signal(self):
        # overridden implementation
        print("D::signal")

c = C()
print(getattr(c.__class__, 'signal'))
print(c.signal)

d = D()
print(getattr(d.__class__, 'signal'))
print(d.signal)

c.signal()    # expect M::signal
d.signal()    # expect D::signal