Qt.ScrollBarAsNeeded not showing scrollbar when it's actually needed

Everything seems to work as expected for me without any need for workarounds. However, I strongly suspect there are additional constraints in your real code that you have not revealed in your question.

UPDATE

Below is a simple example that resizes the scrollareas when the scrollbars are shown/hidden:

import sysfrom PyQt5 import QtCore, QtGui, QtWidgetsclass Window(QtWidgets.QMainWindow):    def __init__(self):        super(Window, self).__init__()        widget = QtWidgets.QWidget(self)        layout = QtWidgets.QHBoxLayout(widget)        self.mdi = QtWidgets.QMdiArea(self)        self.leftScroll = Pane(            QtCore.Qt.AlignTop | QtCore.Qt.AlignLeft, self)        self.rightScroll = Pane(            QtCore.Qt.AlignTop | QtCore.Qt.AlignLeft, self)        layout.addWidget(self.leftScroll)        layout.addWidget(self.mdi)        layout.addWidget(self.rightScroll)        self.setCentralWidget(widget)        for scroll in self.leftScroll, self.rightScroll:            for index in range(4):                widget = QtWidgets.QTextEdit()                widget.setText('one two three four five')                scroll.addWidget(widget)class Pane(QtWidgets.QScrollArea):    MinWidth = 186    def __init__(self, alignment=0, parent=None):        super().__init__(parent)        self.mainWidget = QtWidgets.QWidget(self)        self.mainLayout = QtWidgets.QVBoxLayout(self.mainWidget)        self.mainLayout.setAlignment(alignment)        self.mainLayout.setContentsMargins(0, 0, 0, 0)        self.mainLayout.setSpacing(0)        self.setContentsMargins(0, 0, 0, 0)        self.setFrameStyle(QtWidgets.QFrame.NoFrame)        self.setFixedWidth(Pane.MinWidth)        self.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)        self.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAsNeeded)        self.setSizePolicy(QtWidgets.QSizePolicy.Maximum,                           QtWidgets.QSizePolicy.Ignored)        self.setWidgetResizable(True)        self.setWidget(self.mainWidget)        self.verticalScrollBar().installEventFilter(self)    def addWidget(self, widget):        self.mainLayout.addWidget(widget)    def removeWidget(self, widget):        self.mainLayout.removeWidget(widget)    def eventFilter(self, source, event):        if isinstance(source, QtWidgets.QScrollBar):            if event.type() == QtCore.QEvent.Show:                self.setFixedWidth(Pane.MinWidth + source.width())            elif event.type() == QtCore.QEvent.Hide:                self.setFixedWidth(Pane.MinWidth)        return super(Pane, self).eventFilter(source, event)if __name__ == '__main__':    app = QtWidgets.QApplication(sys.argv)    window = Window()    window.setGeometry(500, 300, 800, 300)    window.show()    sys.exit(app.exec_())