Mate-Desktop - combobox inside groupbox renders selected text in white

Adding a comboxbox to a groupbox causes issues in the rendering of it. It looks like no text is displayed (white text on light grey bg) until it's clicked at which point it opens up and displays the options in black, but once an option is selected, it collapses and reverts back to using white text.

No custom stylesheets used.

Works as expected in KDE.

It's worth noting that adding the combobox directly to the MyWidget layout works as expected.

Sample code:

```
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())
```

To fix, adding a simple stylesheet for Mate-Desktop is a dirty hack:

`
combo.setStyleSheet('*{color:black}')
 
`