X11: QWidget::windowState() ends up at rest out of sync with reality

It is possible, with a quick succession of window state changes, for a Qt program to end up in a situation where (QWidget::windowState() & Qt::WindowMinized) == true even though the native window is not minimized. The following sample reproduces the problem with gnome (mutter):

// Build. g++ -std=c++17 -Wall -Wextra -g -fPIC $(pkg-config --cflags --libs Qt5Core  Qt5Gui Qt5Widgets) -o example example.cc
#include <iostream>
#include <QApplication>
#include <QLabel>
#include <QTimer>

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    QLabel w(QLatin1String("Hello world"));
    w.show();
    QTimer::singleShot(1000, [&] {
        w.setWindowState(w.windowState() | Qt::WindowMinimized);
        w.activateWindow();
    });
    QTimer::singleShot(3000, [&] {
        std::cout << "Window state: " << w.windowState() << std::endl;
        app.quit();
    });
    return app.exec();
}

The program above shows a window with the text "Hello world". After a second it will minimize and then immediately activate the window causing it to not be minimized again. After three seconds it prints the window state and quits. With gnome/mutter this prints "Window state: 1" (minimized) which is incorrect. It should print "Window state: 0".

I have also been able to reproduce the problem with metacity, though not as easily or reliably.

While the example above is synthetic I have observed this problem in a real application.

The problem is that w.setWindowState(w.windowState() | Qt::WindowMinimized); will update QWidget::windowState() directly and then effect the change via the platform QXcbWindow. QWidget::activateWindow() only affects the change via the platform QxcbWindow and relies on property notify events caused by the window manager updating WM_STATE and _NET_WM_STATE to feed the updated window state back to Qt. The problem is that QXcbWindow::handlePropertyNotifyEvent() ignores the updates because it determines that the new state reported is the same as the last update it reported by comparing it to QXcbWindow::m_lastWindowStateEvent.

I believe what goes wrong here is that if window state changes happen fast enough the window manager has changed the WM_STATE/_NET_WM_STATE to minimized and back to unminimized before QXcbWindow::handlePropertyNotifyEvent() runs, which means that from QXcbWindow's point of view there is nothing new to report and we never clear the Qt::WindowMinimized bit that was set directly by QWidget::setWindowState().