How to adapt the size of QWebView to its contents

HtmlEditor should display only a line at first, with the height just enough to hold a line, not bigger, not smaller. As the user inputs more lines, the height of HtmlEditor will get bigger too. I tried to use self.page().mainFrame().contentsSize().height() as follows, it didn't work. I tried to replace self.page().mainFrame().contentsSize().height() with self.fontSize + self.contentsMargins().top() + self.contentsMargins().bottom(). But self.contentsMargins().top() and self.contentsMargins().bottom() are always zero. I don't know why. How to do the trick?

Code:

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class HtmlEditor(QWebView):
    def __init__(self, fontSize):
        super().__init__()
        self.fontSize = fontSize
        self.page().setContentEditable(True)
        self.page().contentsChanged.connect(self.sizeChange)
        self.setupFont()
        self.sizeChange()
 
    def setupFont(self):
        self.htmlEditorSettings = self.settings()
        self.htmlEditorSettings.setFontSize(QWebSettings.DefaultFontSize, self.fontSize)
 
    def sizeChange(self):
        docHeight = self.page().mainFrame().contentsSize().height()
        self.setFixedHeight(docHeight)

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