Adjust window size according content in QScrollArea in PyQt6

Question

I'm codding an app that open json file and according it content fill interface with grid of squares. Base UI I made in QT Designer and it looks like that.

Everything worked good. And I wanted make improvement, such as adjust width window according to content. And after that UI broke. It doesn't adjust, but even shrinks in width.

In debugging I found out that this because of QScrollArea. If I place grid of squares outside this widget, window adjusts fine. But I can't find/understand why QScrollArea doesn't adjust window size.

This is form_ui.py from Qt Designer:

from PyQt6 import QtCore, QtGui, QtWidgets

class Ui_RAL_widget(object):
  def setupUi(self, RAL_widget):
    RAL_widget.setObjectName("RAL_widget")
    RAL_widget.resize(800, 600)
    sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Policy.Expanding, QtWidgets.QSizePolicy.Policy.Expanding)
    sizePolicy.setHorizontalStretch(0)
    sizePolicy.setVerticalStretch(0)
    sizePolicy.setHeightForWidth(RAL_widget.sizePolicy().hasHeightForWidth())
    RAL_widget.setSizePolicy(sizePolicy)
    RAL_widget.setMinimumSize(QtCore.QSize(0, 0))
    RAL_widget.setMaximumSize(QtCore.QSize(800, 600))

    self.horizontalLayout = QtWidgets.QHBoxLayout(RAL_widget)
    self.horizontalLayout.setContentsMargins(-1, -1, 0, -1)
    self.horizontalLayout.setObjectName("horizontalLayout")

    self.RAL_tab = QtWidgets.QTabWidget(parent=RAL_widget)
    self.RAL_tab.setObjectName("RAL_tab")

    self.Classic_tab = QtWidgets.QWidget()
    sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Policy.Expanding, QtWidgets.QSizePolicy.Policy.Expanding)
    sizePolicy.setHorizontalStretch(0)
    sizePolicy.setVerticalStretch(0)
    sizePolicy.setHeightForWidth(self.Classic_tab.sizePolicy().hasHeightForWidth())
    self.Classic_tab.setSizePolicy(sizePolicy)
    self.Classic_tab.setObjectName("Classic_tab")

    self.verticalLayout = QtWidgets.QVBoxLayout(self.Classic_tab)
    self.verticalLayout.setSizeConstraint(QtWidgets.QLayout.SizeConstraint.SetNoConstraint)
    self.verticalLayout.setContentsMargins(0, 0, 0, 0)
    self.verticalLayout.setObjectName("verticalLayout")

    self.classic_tab_scroll_area = QtWidgets.QScrollArea(parent=self.Classic_tab)
    sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Policy.Expanding, QtWidgets.QSizePolicy.Policy.Expanding)
    sizePolicy.setHorizontalStretch(0)
    sizePolicy.setVerticalStretch(0)
    sizePolicy.setHeightForWidth(self.classic_tab_scroll_area.sizePolicy().hasHeightForWidth())
    self.classic_tab_scroll_area.setSizePolicy(sizePolicy)
    self.classic_tab_scroll_area.setAutoFillBackground(False)
    self.classic_tab_scroll_area.setFrameShape(QtWidgets.QFrame.Shape.NoFrame)
    self.classic_tab_scroll_area.setFrameShadow(QtWidgets.QFrame.Shadow.Sunken)
    self.classic_tab_scroll_area.setSizeAdjustPolicy(QtWidgets.QAbstractScrollArea.SizeAdjustPolicy.AdjustToContents)
    self.classic_tab_scroll_area.setWidgetResizable(True)
    self.classic_tab_scroll_area.setAlignment(QtCore.Qt.AlignmentFlag.AlignCenter)
    self.classic_tab_scroll_area.setObjectName("classic_tab_scroll_area")

    self.classic_scroll_widget = QtWidgets.QWidget()
    self.classic_scroll_widget.setGeometry(QtCore.QRect(0, 0, 785, 556))
    sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Policy.Expanding, QtWidgets.QSizePolicy.Policy.Expanding)
    sizePolicy.setHorizontalStretch(0)
    sizePolicy.setVerticalStretch(0)
    sizePolicy.setHeightForWidth(self.classic_scroll_widget.sizePolicy().hasHeightForWidth())
    self.classic_scroll_widget.setSizePolicy(sizePolicy)
    self.classic_scroll_widget.setObjectName("classic_scroll_widget")

    self.classic_grid = QtWidgets.QGridLayout(self.classic_scroll_widget)
    self.classic_grid.setSizeConstraint(QtWidgets.QLayout.SizeConstraint.SetNoConstraint)
    self.classic_grid.setObjectName("classic_grid")
    
    self.classic_tab_scroll_area.setWidget(self.classic_scroll_widget)
    self.verticalLayout.addWidget(self.classic_tab_scroll_area)
    self.RAL_tab.addTab(self.Classic_tab, "")
    self.horizontalLayout.addWidget(self.RAL_tab)

    self.retranslateUi(RAL_widget)
    self.RAL_tab.setCurrentIndex(0)
    QtCore.QMetaObject.connectSlotsByName(RAL_widget)

def retranslateUi(self, RAL_widget):
    _translate = QtCore.QCoreApplication.translate
    RAL_widget.setWindowTitle(_translate("RAL_widget", "RAL Pallete"))
    self.RAL_tab.setTabText(self.RAL_tab.indexOf(self.Classic_tab), _translate("RAL_widget", "Classic"))

And this is my main.py:

import json
import sys
from PyQt6.QtWidgets import QApplication, QWidget, QLabel
from PyQt6.QtCore import Qt
from form_ui import Ui_RAL_widget

class Window(QWidget, Ui_RAL_widget):
  def __init__(self):

    super().__init__()
    self.setupUi(self)
    self.show()

    self.fill_classic_pallete()
    
    # Adjust window width
    self.setFixedSize(self.sizeHint().width(), 600)

    # Debugging
    print(f"Size Hint: {self.sizeHint()}")
    print(f"Actual Size: {self.size()}")


# Fill grid with labels according json data
def fill_classic_pallete(self):
    with open("Ral_classic.json", "r", encoding="utf-8-sig") as ral_file: 
        ral_data = ral_file.read()

        ral_classic = json.loads(ral_data)

        # Columns number in grid
        NUM_COLUMNS = 6

        # Create labels
        for index, ral in enumerate(ral_classic):
            label = QLabel(self.classic_scroll_widget)
            label.setObjectName(ral["RAL"].replace(" ", ""))
            row = index // NUM_COLUMNS
            col = index % NUM_COLUMNS
            label.setText(f"{ral['RAL']}\n{ral['English']}")
            label.setStyleSheet(f"background-color: {ral['HEX']}")
            label.setFixedSize(120, 120)
            label.setAlignment(Qt.AlignmentFlag.AlignCenter)
            self.classic_grid.addWidget(label, row, col)


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

Also here is link to JSON - RAL_classic.json