Vertical centering in QTextEdit is off by 4 pixels

[neuronet]

I have a QTextEdit that I'm using as a single line editor for some tree views: it lets you set the font size and vertical margins for the vertically centered text. The code is pasted below (note it is PySide). I set up the font and margins using a QTextDocument, and then assign that document to the QTextEdit instance. It works great at all margin and font size combinations, but there is one thing driving me nuts: the text is not actually correctly centered on the widget unless I shift it up by four pixels using setViewportMargins. Why?

Could it be related to the fact that QTextDocuments have default margins of 4? I doubt it, because I am manually setting the margins of the document.

Any ideas?

(Before you ask, I'm not using QLineEdit because I'm displaying html in my tree views...)

SSCCE

import sys
from PySide import QtGui, QtCore
 
class TextLineEdit(QtGui.QTextEdit):
    topMarginCorrection = -4 #not sure why needed    
    returnPressed = QtCore.Signal()
    def __init__(self, fontSize = 10, verticalMargin = 2, parent = None):
        QtGui.QTextEdit.__init__(self, parent)
        self.setAttribute(QtCore.Qt.WA_DeleteOnClose) 
        self.setLineWrapMode(QtGui.QTextEdit.NoWrap)
        self.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.setFontPointSize(fontSize)
        self.setViewportMargins(-verticalMargin, self.topMarginCorrection , 0, 0)  #left, top, right, bottom
        #Set up document with appropriate margins and font
        document = QtGui.QTextDocument()
        currentFont = self.currentFont()
        currentFont.setPointSize(fontSize)
        document.setDefaultFont(currentFont)
        document.setDocumentMargin(verticalMargin)  
        self.setFixedHeight(document.size().height())
        self.setDocument(document)
 
    def keyPressEvent(self, event):
        '''stops return from returning newline'''
        if event.key() in (QtCore.Qt.Key_Enter, QtCore.Qt.Key_Return):
            self.returnPressed.emit()
            event.accept()
        else:
            QtGui.QTextEdit.keyPressEvent(self, event)
 
 
def main():
    app = QtGui.QApplication(sys.argv)
    myLine = TextLineEdit(fontSize = 15, verticalMargin = 8)
    myLine.show()    
    sys.exit(app.exec_())
 
 
if __name__ == "__main__":
    main()

import sys
from PySide import QtGui, QtCore
         
class TextLineEdit(QtGui.QTextEdit):
    topMarginCorrection = -4 #not sure why needed    
    returnPressed = QtCore.Signal()
    def __init__(self, fontSize = 10, verticalMargin = 2, parent = None):
        QtGui.QTextEdit.__init__(self, parent)
        self.setAttribute(QtCore.Qt.WA_DeleteOnClose) 
        self.setLineWrapMode(QtGui.QTextEdit.NoWrap)
        self.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.setFontPointSize(fontSize)
        self.setViewportMargins(-verticalMargin, self.topMarginCorrection , 0, 0)  #left, top, right, bottom
        #Set up document with appropriate margins and font
        document = QtGui.QTextDocument()
        currentFont = self.currentFont()
        currentFont.setPointSize(fontSize)
        document.setDefaultFont(currentFont)
        document.setDocumentMargin(verticalMargin)  
        self.setFixedHeight(document.size().height())
        self.setDocument(document)

    def keyPressEvent(self, event):
        '''stops return from returning newline'''
        if event.key() in (QtCore.Qt.Key_Enter, QtCore.Qt.Key_Return):
            self.returnPressed.emit()
            event.accept()
        else:
            QtGui.QTextEdit.keyPressEvent(self, event)

            
def main():
    app = QtGui.QApplication(sys.argv)
    myLine = TextLineEdit(fontSize = 15, verticalMargin = 8)
    myLine.show()    
    sys.exit(app.exec_())
    
    
if __name__ == "__main__":
    main()

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