The full code:

from PySide6.QtWidgets import QScrollArea, QDialog, QComboBox, QLabel, QGroupBox, QCheckBox, QPushButton, QGridLayout, QVBoxLayout, QHBoxLayout, QSpacerItem, QSizePolicy, QFormLayout
from PySide6.QtSerialPort import QSerialPortInfo, QSerialPort
from PySide6.QtCore import Qt,QTimer,Signal
from PySide6.QtGui import QFont, QIcon

from dataclasses import dataclass

@dataclass
class SerialSettings:
    name: str
    baudRate: int
    stringBaudRate: str
    dataBits: int
    stringDataBits: str
    parity: int
    stringParity: str
    stopBits: int
    stringStopBits: str
    flowControl: int
    stringFlowControl: str




class SettingsWindow(QDialog):
    settings_applied = Signal(str)  # New signal to indicate the directory currently being downloaded
    def __init__(self, parent=None):
        super(SettingsWindow, self).__init__(parent)
        self.setWindowTitle("Settings")
        self.setModal(True)  # This makes the dialog modal
        self.setWindowFlags(Qt.Dialog | Qt.WindowCloseButtonHint | Qt.FramelessWindowHint)

        # Layouts
        self.setFixedWidth(300)  # Set the desired width in pixels

        self.mainLayout = QVBoxLayout(self)
        self.setLayout(self.mainLayout)



        self.createParameterBox()
        self.createSelectBox()
        self.createApplyButton()

        # Apply unified stylesheet
        self.setStyleSheet(self.getStyleSheet())

        # Set font for the entire window
        font = QFont("Arial", 10)
        self.setFont(font)

        self.applyButton.clicked.connect(self.apply)
        self.closeButton.clicked.connect(self.reject)
        self.ui_serialPortInfoListBox.currentIndexChanged.connect(self.showPortInfo)

        self.fillPortsParameters()
        self.fillPortsInfo()
        self.updateSettings()

    def createParameterBox(self):
        self.parametersBox = QGroupBox("Select Parameters")
        self.parametersLayout = QFormLayout(self.parametersBox)

        self.ui_baudRateBox = QComboBox()
        self.ui_dataBitsBox = QComboBox()
        self.ui_parityBox = QComboBox()
        self.ui_stopBitsBox = QComboBox()
        self.ui_flowControlBox = QComboBox()

        self.parametersLayout.addRow(QLabel("BaudRate:"), self.ui_baudRateBox)
        self.parametersLayout.addRow(QLabel("Data bits:"), self.ui_dataBitsBox)
        self.parametersLayout.addRow(QLabel("Parity:"), self.ui_parityBox)
        self.parametersLayout.addRow(QLabel("Stop bits:"), self.ui_stopBitsBox)
        self.parametersLayout.addRow(QLabel("Flow control:"), self.ui_flowControlBox)

        self.mainLayout.addWidget(self.parametersBox)



    def createSelectBox(self):
        # Create the group box for serial port selection
        self.selectBox = QGroupBox("Select Serial Port")
        self.selectLayout = QFormLayout(self.selectBox)

        # Create UI components
        self.ui_serialPortInfoListBox = QComboBox(self)
        self.descriptionLabel = QLabel("Description:")
        self.manufacturerLabel = QLabel("Manufacturer:")
        self.serialNumberLabel = QLabel("Serial Number:")
        self.locationLabel = QLabel("Location:")
        self.vidLabel = QLabel("Vendor ID:")
        self.pidLabel = QLabel("Product ID:")

        # Add components to the form layout within the group box
        self.selectLayout.addRow(self.ui_serialPortInfoListBox)
        self.selectLayout.addRow(self.descriptionLabel)
        self.selectLayout.addRow(self.manufacturerLabel)
        self.selectLayout.addRow(self.serialNumberLabel)
        self.selectLayout.addRow(self.locationLabel)
        self.selectLayout.addRow(self.vidLabel)
        self.selectLayout.addRow(self.pidLabel)

        # Create a scroll area and set the group box as its widget
        self.scrollArea = QScrollArea(self)
        self.scrollArea.setWidgetResizable(True)  # Allows resizing of the content
        self.scrollArea.setWidget(self.selectBox)  # Set the group box as the content of the scroll area

        # Add the scroll area to the main layout instead of the group box directly
        self.mainLayout.addWidget(self.scrollArea)

    def createApplyButton(self):
        self.horizontalLayout = QHBoxLayout()

        self.applyButton = QPushButton("Apply")
        self.applyButton.setIcon(QIcon.fromTheme("dialog-ok-apply"))

        self.closeButton = QPushButton("Close")
        self.closeButton.setIcon(QIcon.fromTheme("dialog-close"))

        self.horizontalLayout.addWidget(self.applyButton)
        self.horizontalLayout.addWidget(self.closeButton)


        self.mainLayout.addLayout(self.horizontalLayout)




    def getStyleSheet(self):
        return """
        QDialog {
            background-color: #242C34;
            color: #FFFFFF;
        }
        QGroupBox {
            background-color: #3B4252;
            border: 1px solid #4C566A;
            border-radius: 5px;
            margin-top: 20px;
        }
        QGroupBox::title {
            color: #FFFFFF;
            subcontrol-origin: margin;
            subcontrol-position: top center;
            padding: 10px;
        }
        QLabel {
            color: #D8DEE9;
        }
        QComboBox, QCheckBox, QPushButton {
            background-color: #8A9C97;
            color: #D8DEE9;
            border: 1px solid #5E81AC;
            border-radius: 3px;
            padding: 5px;
        }
        QPushButton {
            background-color: #8A9C97;
            padding: 10px;
            font-weight: bold;
        }
        QPushButton:hover {
            background-color: #8FA047;
        }
        QComboBox:hover {
            background-color: #8FA047;
        }
        QComboBox QAbstractItemView {
            background-color: #434C5E;
            selection-background-color: #81A1C1;
            color: white;
        }





        """

    def showPortInfo(self, idx):
        if idx == -1:
            return
        data = self.ui_serialPortInfoListBox.itemData(idx)
        if data is None:
            self.descriptionLabel.setText("Description: N/A")
            self.manufacturerLabel.setText("Manufacturer: N/A")
            self.serialNumberLabel.setText("Serial Number: N/A")
            self.locationLabel.setText("Location: N/A")
            self.vidLabel.setText("Vendor ID: N/A")
            self.pidLabel.setText("Product ID: N/A")
            return
        self.descriptionLabel.setText(f"Description: {data[1]}")
        self.manufacturerLabel.setText(f"Manufacturer: {data[2]}")
        self.serialNumberLabel.setText(f"Serial Number: {data[3]}")
        self.locationLabel.setText(f"Location: {data[4]}")
        self.vidLabel.setText(f"Vendor ID: {data[5]}")
        self.pidLabel.setText(f"Product ID: {data[6]}")

    def apply(self):
        self.updateSettings()
        self.settings_applied.emit(self.m_currentSettings.name)
        self.accept()

    def fillPortsParameters(self):
        # Note that QSerialPort's baud rate constants are used here as item data.
        self.ui_baudRateBox.addItem("115200", QSerialPort.Baud115200)
        self.ui_baudRateBox.addItem("9600", QSerialPort.Baud9600)
        self.ui_baudRateBox.addItem("19200", QSerialPort.Baud19200)
        self.ui_baudRateBox.addItem("38400", QSerialPort.Baud38400)
        self.ui_baudRateBox.addItem("Custom", None)  # This is for custom baud rate, assuming None is intentional

        self.ui_dataBitsBox.addItem("5", QSerialPort.Data5)
        self.ui_dataBitsBox.addItem("6", QSerialPort.Data6)
        self.ui_dataBitsBox.addItem("7", QSerialPort.Data7)
        self.ui_dataBitsBox.addItem("8", QSerialPort.Data8)
        self.ui_dataBitsBox.setCurrentIndex(3)


        self.ui_parityBox.addItem("None", QSerialPort.NoParity)
        self.ui_parityBox.addItem("Even", QSerialPort.EvenParity)
        self.ui_parityBox.addItem("Odd", QSerialPort.OddParity)
        self.ui_parityBox.addItem("Mark", QSerialPort.MarkParity)
        self.ui_parityBox.addItem("Space", QSerialPort.SpaceParity)


        self.ui_stopBitsBox.addItem("1", QSerialPort.OneStop)
        self.ui_stopBitsBox.addItem("2", QSerialPort.TwoStop)

        self.ui_flowControlBox.addItem("None", QSerialPort.NoFlowControl)
        self.ui_flowControlBox.addItem("RTS/CTS", QSerialPort.HardwareControl)
        self.ui_flowControlBox.addItem("XON/XOFF", QSerialPort.SoftwareControl)


    def fillPortsInfo(self):
        self.ui_serialPortInfoListBox.clear()
        infos = QSerialPortInfo.availablePorts()
        for info in infos:
            data = [
                info.portName(),
                info.description(),
                info.manufacturer(),
                info.serialNumber(),
                info.systemLocation(),
                hex(info.vendorIdentifier()) if info.vendorIdentifier() else "N/A",
                hex(info.productIdentifier()) if info.productIdentifier() else "N/A",
            ]
            self.ui_serialPortInfoListBox.addItem(data[0], data)
        self.ui_serialPortInfoListBox.addItem("Custom", None)

    def updateSettings(self):
        current_index = self.ui_baudRateBox.currentIndex()
        baudRate = int(self.ui_baudRateBox.currentText()) if current_index == 4 else self.ui_baudRateBox.itemData(current_index)
        self.m_currentSettings = SerialSettings(
            name=self.ui_serialPortInfoListBox.currentText(),
            baudRate=baudRate,
            stringBaudRate=self.ui_baudRateBox.currentText(),
            dataBits=self.ui_dataBitsBox.currentData(),
            stringDataBits=self.ui_dataBitsBox.currentText(),
            parity=self.ui_parityBox.currentData(),
            stringParity=self.ui_parityBox.currentText(),
            stopBits=self.ui_stopBitsBox.currentData(),
            stringStopBits=self.ui_stopBitsBox.currentText(),
            flowControl=self.ui_flowControlBox.currentData(),
            stringFlowControl=self.ui_flowControlBox.currentText(),
        )

    def settings(self):
        return self.m_currentSettings

It creates the following window:

I am really frustrated that there is this blank space out there for no reason and I cannot find a way to get rid of it. I would appreciate if someone could help me out here! I use PySide6