[Python] How to fix PySide6 overlay widget showing through collapsed QSplitter sidebar?

I am trying to build a simple nice looking renderer to make some Graphs for future Projects, therefore I have an overlay with buttons positioned in the top corner of a renderer widget. The renderer is vtk by the way, just if there is a known Problem. When I collapse a sidebar using QSplitter, the overlay shows through the collapsed sidebar, so I see the sidebar, which should be collapsed. I have attached a simplified example of my Problem.

from PySide6.QtWidgets import (
QApplication, QMainWindow, QWidget, QSplitter,
QToolButton, QBoxLayout
)
from PySide6.QtCore import Qt
import vtk
from vtk.qt.QVTKRenderWindowInteractor import QVTKRenderWindowInteractor

class Renderer:
def __init__(self, parent=None):
# Create the widget and renderer
self.plotter = QVTKRenderWindowInteractor(parent)
self.renderer = vtk.vtkRenderer()
self.renderer.SetBackground(0, 0, 0)

# Set up the render window
render_window = self.plotter.GetRenderWindow()
render_window.AddRenderer(self.renderer)

# Create interactor style
self.style = vtk.vtkInteractorStyleTrackballCamera()
self.plotter.SetInteractorStyle(self.style)
self.plotter.Initialize()



class MainWindow(QMainWindow):
def __init__(self):
super().__init__()
self.setWindowTitle("Overlay Issue")

# Create main splitter
self.main_splitter = QSplitter(Qt.Horizontal)
self.left_sidebar = QWidget()
self.left_sidebar.setStyleSheet("background-color: #303840;")
self.left_sidebar.setMinimumWidth(200)

self.renderer = Renderer(self)

self.main_splitter.addWidget(self.left_sidebar)
self.main_splitter.addWidget(self.renderer.plotter)
self.main_splitter.setSizes([300, 600, 300])

self.setCentralWidget(self.main_splitter)

# Create overlay with buttons
self.overlay = QWidget(self.renderer.plotter)
self.overlay.setStyleSheet("background: transparent;")

h = QBoxLayout(QBoxLayout.LeftToRight)
self.overlay.setLayout(h)

btn = QToolButton()
btn.setText('text')
btn.setCheckable(True)
btn.setStyleSheet("background-color: #0078d4; color: white; border: none;")
h.addWidget(btn)

self.overlay.adjustSize()
self.overlay.move(10, 10)


if __name__ == "__main__":
app = QApplication([])
window = MainWindow()
window.show()
app.exec()


I have tried to replace the vtk renderer with with a QWidget, and it seems that the Problem then dissapears.

Because on resizing the entire window the problem goes away I tried to call a resize event, which I did not been able to make it work

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