Bug#1059530: mate-desktop: Pyside6 combobox inside a groupbox doesn't render selected option but does whilst window is resizing

[Peter Thompson <theoneandonly@gmx.us>]

Package: mate-desktop
Version: 1.26.2-1
Severity: important
X-Debbugs-Cc: theoneandonly@gmx.us

Dear Maintainer,

Creating Qt GUI in python for a project and discovered the problem. 

Minimum python code below. I've tested in KDE and it works as expected.

Python 3.11

```
from PySide6.QtWidgets import QApplication, QFrame, QGroupBox, QComboBox, QWidget, QVBoxLayout
import sys

class MyWidget(QWidget):
    def __init__(self):
        super().__init__()

        groupbox = QGroupBox(self)
        groupbox_layout = QVBoxLayout(groupbox)
        
        combo = QComboBox(groupbox)
        combo.addItems(["Option 1", "Option 2", "Option 3"])
        groupbox_layout.addWidget(combo)

        layout = QVBoxLayout(self)
        layout.addWidget(groupbox)

app = QApplication(sys.argv)
window = MyWidget()
window.show()
sys.exit(app.exec())
```

-- System Information:
Debian Release: trixie/sid
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'unstable'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 6.6.8-amd64 (SMP w/8 CPU threads; PREEMPT)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages mate-desktop depends on:
ii  hicolor-icon-theme    0.17-2
ii  libc6                 2.37-13
ii  libglib2.0-0          2.78.3-1
ii  libgtk-3-0            3.24.39-1
ii  libmate-desktop-2-17  1.26.2-1
ii  mate-desktop-common   1.26.2-1

Versions of packages mate-desktop recommends:
ii  mate-user-guide  1.26.2-1

Versions of packages mate-desktop suggests:
ii  mate-desktop-environment  1.26.0+1

-- no debconf information