Inconsistent borders when drawing a custom translucent widget using QPainterPath

[ParikshitT.]

Hi,
I am trying to draw a widget with a triangular tip at the bottom with the help of this guide. The shape is drawn as expected but trying to add a border to it makes it sag from the bottom:

This is my first attempt at making a custom shaped widget and would appreciate if someone could help me in figuring this out :)
Please note that the code is in python but I do not mind looking at C++ sources and documentation.

import typing as _t

from Qt import (
    QtCore as _QtCore,
    QtGui as _QtGui,
    QtWidgets as _QtWidgets,
)


class _TipTriangleDimensions(_t.NamedTuple):
    height: float = 15.0
    base: float = 20.0


class _TipTriangleCords(_t.NamedTuple):
    origin: _QtCore.QPointF
    tip: _QtCore.QPointF
    base: _QtCore.QPointF


class QBalloonTip(_QtWidgets.QWidget):
    def __init__(
        self, message: str, parent: _t.Optional[_QtWidgets.QWidget] = None
    ) -> None:
        super().__init__(
            parent=parent,
            f=_QtCore.Qt.FramelessWindowHint,
        )
        self.setAttribute(_QtCore.Qt.WA_TranslucentBackground)
        main_layout = _QtWidgets.QVBoxLayout()
        self._setup_ui(main_layout)
        self._message_label.setText(message)
        self._mask_path: _QtGui.QPainterPath = self._get_mask_shape_path(
            _TipTriangleDimensions()
        )

    def paintEvent(self, event):
        painter = _QtGui.QPainter(self)
        painter.setRenderHint(_QtGui.QPainter.RenderHint.Antialiasing)
        painter.setBrush(_QtGui.QBrush(_QtCore.Qt.white))
        border_pen = _QtGui.QPen()
        border_pen.setWidthF(6.0)
        border_pen.setColor(_QtCore.Qt.red)
        painter.setPen(border_pen)
        painter.drawPath(self._mask_path)

    def _get_mask_shape_path(self, triangle_d: _TipTriangleDimensions) -> _QtGui.QPainterPath:
        self.setContentsMargins(0, 0, 0, int(triangle_d.height))
        self.updateGeometry()
        self_sz = _QtCore.QSizeF(self.sizeHint())

        triangle_cords = self._get_triangle_shape_points(self_sz, triangle_d)

        origin = _QtCore.QPointF(0.0, 0.0)
        right = _QtCore.QPointF(self_sz.width(), triangle_cords.base.y())
        top = _QtCore.QPointF(self_sz.width(), origin.y())

        path = _QtGui.QPainterPath()
        path.moveTo(origin)
        path.lineTo(triangle_cords.tip)
        path.lineTo(triangle_cords.base)
        path.lineTo(right)
        path.lineTo(top)
        path.lineTo(origin)
        return path

    @staticmethod
    def _get_triangle_shape_points(
        total_size: _QtCore.QSizeF, dimensions: _TipTriangleDimensions
    ) -> _TipTriangleCords:
        origin = _QtCore.QPointF(0.5, total_size.height() - dimensions.height)
        tip = _QtCore.QPointF(0.0, total_size.height())
        base = _QtCore.QPointF(dimensions.base, origin.y())
        return _TipTriangleCords(origin, tip, base)

    def _setup_ui(self, main_layout: _QtWidgets.QVBoxLayout):
        self._message_label = _QtWidgets.QLabel(parent=self)
        self._message_label.setFixedWidth(200)
        self._message_label.setFixedHeight(100)
        self._message_label.setWordWrap(True)

        main_layout.addWidget(self._message_label)
        main_layout.addWidget(_QtWidgets.QPushButton("Test"))
        self.setLayout(main_layout)


if __name__ == "__main__":
    app = _QtWidgets.QApplication([])
    widget = QBalloonTip("test message")
    widget.show()
    app.exec_()