Not strictly through designer no, there’s no simple checkbox to enable it.
Here’s some simple code to enable the frame-less window, and set up a masking image in resizeEvent. I know you wanted to do it in designer, but seeing as that’s not possible right now I figured I’d give you something useful instead
#in init: self.setWindowFlags(QtCore.Qt.FramelessWindowHint)
#Make a rounded 8px rectangle to mask out the window
def resizeEvent(self, event):
pixmap = QtGui.QPixmap(self.size())
pixmap.fill(QtCore.Qt.transparent)
painter = QtGui.QPainter(pixmap)
painter.setBrush(QtCore.Qt.black)
painter.drawRoundedRect(pixmap.rect(), 8, 8)
painter.end()
self.setMask(pixmap.mask())