奇怪的问题，部分快捷键失效，QPlainTextEdit有bug吗？

Question

按照网上打造了一个代码编辑器，奇怪的是，下拉菜单粘贴和键盘ctrl+v有效，而下拉菜单复制和键盘ctrl+c无效，我没有写任何这方面的代码，难道是QPlainTextEdit的bug？
class QPlainTextEditLineNum(QPlainTextEdit):

def __init__(self, parent=None):
    super().__init__(parent)
    self.setLineWrapMode(QPlainTextEdit.NoWrap)  # 不自动换行
    self.lineNumberArea = LineNumPaint(self)
    self.document().blockCountChanged.connect(self.update_line_num_width)
    self.document().cursorPositionChanged.connect(self.highlightCurrentLine)    # 高亮当前行
    self.verticalScrollBar().sliderMoved.connect(self.scroll_event)  # 滚动条移动更新行号
    self.update_line_num_width()
    self.highlightCurrentLine()

def highlightCurrentLine(self):
    extraSelections = QTextEdit.extraSelections(QTextEdit())
    if not self.isReadOnly():
        selection = QTextEdit.ExtraSelection()
        lineColor = QColor(Qt.yellow).lighter(160)
        selection.format.setBackground(lineColor)
        selection.format.setProperty(QTextFormat.FullWidthSelection, True)
        selection.cursor = self.textCursor()
        selection.cursor.clearSelection()
        extraSelections.append(selection)
    self.setExtraSelections(extraSelections)
def wheelEvent(self, d: QWheelEvent) -> None:
    self.scroll_event(d)
    super().wheelEvent(d)

def scroll_event(self, event: QWheelEvent=None):
    self.lineNumberArea.update()

def keyPressEvent(self, e: QKeyEvent) -> None:
    super().keyPressEvent(e)
    self.lineNumberArea.update()

def mousePressEvent(self, e: 'QMouseEvent') -> None:
    super().mousePressEvent(e)
    self.update()
    self.highlightCurrentLine()

def lineNumberAreaWidth(self):
    block_count = self.document().blockCount()
    max_value = max(1, block_count)
    d_count = len(str(max_value))
    _width = self.fontMetrics().width('9') * d_count + 5
    return _width

def update_line_num_width(self):
    self.setViewportMargins(self.lineNumberAreaWidth(), 0, 0, 0)

def resizeEvent(self, event):
    super().resizeEvent(event)
    cr = self.contentsRect()
    self.lineNumberArea.setGeometry(QRect(cr.left(), cr.top(), self.lineNumberAreaWidth(), cr.height()))

def lineNumberAreaPaintEvent(self, event):
    cursor = QTextCursor(self.document())
    painter = QPainter(self.lineNumberArea)

    painter.fillRect(event.rect(), Qt.lightGray)
    line_height = self.fontMetrics().lineSpacing()  # 包含行间距的行高

    block_number = self.cursorForPosition(QPoint(0, int(line_height / 2))).blockNumber()
    first_visible_block = self.document().findBlock(block_number)
    blockNumber = block_number
    cursor.setPosition(self.cursorForPosition(QPoint(0, int(line_height / 2))).position())
    rect = self.cursorRect()
    scroll_compensation = rect.y() - int(rect.y() / line_height) * line_height
    top = scroll_compensation
    last_block_number = self.cursorForPosition(QPoint(0, self.height() - 1)).blockNumber()

    height = self.fontMetrics().height()
    block = first_visible_block
    while block.isValid() and (top <= event.rect().bottom()) and blockNumber <= last_block_number:
        # cur_line_count = block.lineCount()
        if block.isVisible():
            number = str(blockNumber + 1)
            painter.setPen(Qt.black)
            # print((0, top, self.lineNumberArea.width(), height), number)
            painter.drawText(0, top, self.lineNumberArea.width(), height, Qt.AlignCenter, number)
        block = block.next()
        top = top + line_height
        blockNumber += 1

class LineNumPaint(QWidget):

def __init__(self, q_edit):
    super().__init__(q_edit)
    self.q_edit_line_num = q_edit

def sizeHint(self):
    return QSize(self.q_edit.lineNumberAreaWidth(), 0)

def paintEvent(self, event):
    self.q_edit_line_num.lineNumberAreaPaintEvent(event)

谢谢大神回复。
代码就是从网上复制来的，小白一个，暂时解决不了问题，继续求助网络
补充问题：
cut CTRL+X键页可以删去选择文本，就是系统剪切板内容没有变化，系统剪切板只可读不可写，有写保护系变量要打开吗？

一直没有留意，发现copy时出现
OleSetClipboard: Failed to set mime data (text/plain, text/html, application/vnd.oasis.opendocument.text) on clipboard: COM error 0xffffffff800401f0 CO_E_NOTINITIALIZED (Unknown error 0x0800401f0) (参数错误。)
有说是
它不能在子线程或者非gui线程下用，怎么办？
有说

看到这样一段网文，不知道怎样做，太小白了
文本的选择由QTextCursor类处理，该类提供创建选择，检索文本内容或删除选择的功能。您可以使用textCursor()方法检索与用户可见光标对应的对象。如果你想在QPlainTextEdit中设置一个选择，只需在QTextCursor对象上创建一个选择，然后使用setCursor()将该游标设置为可见光标。选择可以通过copy()复制到剪贴板，也可以用cut()剪切到剪贴板。