import sys

from PyQt5.QtCore import Qt
from PyQt5.QtWidgets import QApplication, QScrollArea, QPushButton, QFrame, QSplitter

app = QApplication([])

button = QPushButton('button')
button.setMinimumSize(200, 400)

scroll = QScrollArea()
scroll.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
scroll.setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOn)
scroll.setWidget(button)

splitter = QSplitter()
splitter.setChildrenCollapsible(False)

splitter.addWidget(scroll)
splitter.addWidget(QFrame())

splitter.show()

# Should happen automatically
# scroll.setMinimumWidth(200 + scroll.verticalScrollBar().width())

sys.exit(app.exec())