With PyQt4, why does setting a tabwidget tabbar icon widget to a label with an animated QMovie sometimes cause the animation to pause? [With Testcase]

Question

I have a QMainWindow with a QTabWidget. From the QTabWidget, you can get a QTabBar and the set the QTabBar's TabButton as per https://doc.qt.io/archives/qt-4.8/qtabbar.html#setTabButton

So, I'd like to do this to put a throbber (animated gif) next to the text for each tab. This "usually" worked fine, until sometimes a peculiar behavior occured - namely, the animation would freeze and not update. Then, moving your mouse lets the animation update. Clearly there was as issue wit the event processing - if Qt has an event then the animation worked fine, but if not, it paused.

I've finally managed to get a reduced testcase for this problem after a days work, and it's listed below.

Copy the below code into a file, create an 'images' subfolder underneath it, and throw in a throbber.gif (say http://upload.wikimedia.org/wikipedia/en/7/78/Netscape_throbber_2.gif ).

Run the program, and notice how the throbber only animates when you move your mouse over the window. Next, change the line EVILNESS=10 to EVILNESS=1 and rerun the program. Notice now how the throbber animates continuously, even without moving your mouse.

Why is this? can I work around it? Am I doing something wrong?

My system:

Windows XP SP3 Python 2.6.5 (r265:79096, Mar 19 2010, 21:48:26) [MSC v.1500 32 bit (Intel)] PyQt 4.7.4

Thanks for the help / suggestions.

import os, sys
from PyQt4 import QtCore, QtGui

GOODNESS = 10
EVILNESS = 10

class MyWindow(QtGui.QMainWindow):
    def __init__(self, app):
        QtGui.QMainWindow.__init__(self)
        self.app = app
        self.initUI()
        for i in range(GOODNESS):
            self.addTab()
        for i in range(EVILNESS):
            self.animateTabIndex(self.tabMain, i, True)

    def initUI(self):
        self.centralwidget=QtGui.QWidget(self)
        self.tabMain = QtGui.QTabWidget(self.centralwidget)
        self.tabOne = QtGui.QWidget()
        self.tabOne.edit = QtGui.QLineEdit(self.tabOne)
        self.tabOne.edit.setText(QtCore.PYQT_VERSION_STR) #Qt Version
        self.tabOneLayout = QtGui.QVBoxLayout(self.tabOne)
        self.tabOneLayout.addWidget(self.tabOne.edit)
        self.tabMain.addTab(self.tabOne, "First Tab")
        self.verticalLayout = QtGui.QVBoxLayout(self.centralwidget)
        self.verticalLayout.addWidget(self.tabMain)
        self.setCentralWidget(self.centralwidget)

    def addTab(self):
        et = QtGui.QWidget()
        someedit = QtGui.QLineEdit(et)
        somelayout = QtGui.QVBoxLayout(et)
        somelayout.addWidget(someedit)
        self.tabMain.addTab(et, "Extra Tab")

    def animateTabIndex(self, tabWidget, tabIndex, enable):
        print tabIndex
        tabBar = tabWidget.tabBar()
        if enable:
            lbl = QtGui.QLabel(tabWidget)
            movie = QtGui.QMovie(os.path.join(self.app.basedir, "images\\throbber.gif"), parent=lbl)
            movie.setCacheMode(QtGui.QMovie.CacheAll)
            movie.setScaledSize(QtCore.QSize(16, 16))
            lbl.setMovie(movie)
            movie.start()
        else:
            lbl = QtGui.QLabel(tabWidget)
            lbl.setMinimumSize(QtCore.QSize(16, 16))
        tabBar.setTabButton(tabIndex, QtGui.QTabBar.LeftSide, lbl)

class MyApp(QtGui.QApplication):
    basedir = os.path.dirname(__file__)
    def __init__(self, args):
        super(MyApp, self).__init__(args)
        self.mainWindow = MyWindow(self)
    def exec_(self):
        QtCore.pyqtRemoveInputHook() #Needed to allow pdb, etc to work
        ret = super(MyApp, self).exec_()
        return ret

if __name__ == '__main__':
    app = MyApp(sys.argv)
    app.mainWindow.show()
    app.exec_()

Answer 1

Well, this was apparently fixed by PyQt 4.8.2. Guess I should have tried upgrading earlier...

UPDATE: Specifically this: http:/bugreports.qt-project.org/browse/QTBUG-12721