Click based screen grab workaround for systems not supporting Qt mouseGrab()

grabtest.py

from PyQt5 import QtCore, QtGui, QtWidgets


class FakeCursor(QtWidgets.QWidget):
    def __init__(self):
        super().__init__()
        self.setWindowFlags(QtCore.Qt.FramelessWindowHint | QtCore.Qt.ToolTip)
        self.setFixedSize(16, 16)
        path = QtGui.QPainterPath()
        path.moveTo(0, 8)
        path.lineTo(6, 8)
        path.moveTo(8, 0)
        path.lineTo(8, 6)
        path.moveTo(10, 8)
        path.lineTo(16, 8)
        path.moveTo(8, 10)
        path.lineTo(8, 16)
        stroker = QtGui.QPainterPathStroker()
        stroker.setCapStyle(QtCore.Qt.FlatCap)
        stroker.setWidth(2)
        final = stroker.createStroke(path)
        region = QtGui.QRegion()
        for path in final.toFillPolygons(QtGui.QTransform()):
            region |= QtGui.QRegion(path.toPolygon())
        self.setMask(region)

    def closeEvent(self, event):
        event.ignore()

    def paintEvent(self, event):
        qp = QtGui.QPainter(self)
        qp.fillRect(self.rect(), QtCore.Qt.black)


class GrabTest(QtWidgets.QWidget):
    def __init__(self):
        super().__init__()
        layout = QtWidgets.QVBoxLayout(self)
        self.grabButton = QtWidgets.QPushButton('Grab')
        layout.addWidget(self.grabButton)
        self.grabButton.installEventFilter(self)
        self.grabButton.setCheckable(True)
        self.grabButton.toggled.connect(self.startGrab)

        self.label = QtWidgets.QLabel('Waiting')
        layout.addWidget(self.label)

        self.mousePos = None
        self.fakeCursor = FakeCursor()

    def startGrab(self, state):
        if state:
            # store the cursor position
            self.mousePos = QtGui.QCursor.pos()
            self.grabButton.setMouseTracking(True)
            self.grabButton.grabMouse()
            QtWidgets.QApplication.setOverrideCursor(QtGui.QCursor(QtCore.Qt.BlankCursor))
            self.fakeCursor.show()
            self.fakeCursor.move(self.mousePos - self.fakeCursor.rect().center())
            self.label.setText('Grabbing')
        else:
            self.grabButton.setMouseTracking(False)
            self.grabButton.releaseMouse()
            self.grabButton.blockSignals(True)
            self.grabButton.setChecked(False)
            self.grabButton.blockSignals(False)
            QtGui.QCursor.setPos(self.fakeCursor.pos() + self.fakeCursor.rect().center())
            self.mousePos = None
            QtWidgets.QApplication.restoreOverrideCursor()
            self.fakeCursor.hide()

    def getPixel(self):
        desktopId = QtWidgets.QApplication.desktop().winId()
        screenshot = QtWidgets.QApplication.primaryScreen().grabWindow(desktopId).toImage()
        pos = self.fakeCursor.pos() + self.fakeCursor.rect().center()
        pixel = screenshot.pixelColor(pos)
        self.label.setText('Click on {}x{}\nRGB: {}'.format(pos.x(), pos.y(), pixel.name()))

    def eventFilter(self, source, event):
        if source == self.grabButton:
            if event.type() == QtCore.QEvent.MouseButtonPress and self.grabButton.isChecked():
                self.getPixel()
            elif event.type() == QtCore.QEvent.MouseButtonRelease and event.button() == QtCore.Qt.LeftButton:
                self.grabButton.toggle()
                return True
            elif event.type() == QtCore.QEvent.MouseMove and self.mousePos is not None:
                # get the difference with the previous mouse position
                delta = event.globalPos() - self.mousePos

                screenRect = QtCore.QRect()
                for screen in QtWidgets.QApplication.screens():
                    screenRect |= screen.geometry()

                # set the (possible) geometry using the delta
                geo = self.fakeCursor.geometry().translated(delta)
                # ensure that the fake cursor doesn't go outside screen margins
                center = geo.center()
                diff = self.fakeCursor.width() // 2
                if center.x() > screenRect.right():
                    geo.moveRight(screenRect.right() + diff)
                elif center.x() < screenRect.left():
                    geo.moveLeft(screenRect.left() - diff)
                if center.y() > screenRect.bottom():
                    geo.moveBottom(screenRect.bottom() + diff)
                elif center.y() < screenRect.top():
                    geo.moveTop(screenRect.top() - diff)

                # actually move the fake cursor
                self.fakeCursor.move(geo.topLeft())
                # reset the cursor position back to the initial
                QtGui.QCursor.setPos(self.mousePos)
        return super().eventFilter(source, event)

if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = GrabTest()
    w.show()
    sys.exit(app.exec_())