Bug 440751

Note that keyboard repeat rate could be very low to trigger this bug, so it doesn't seem like race condition. The video has repeat rate of 3 per second.

I can confirm this. I just started seeing it yesterday on Wayland and found that I could reproduce it pretty reliably by showing the Widget Explorer (which automatically invokes Present Windows), dragging a widget to the desktop, and interacting with the desktop at all. Thereafter, the Present Windows effect has restored all the windows, but they are invisible.

If I quit an affected app and re-launch it, its window is rendered properly.

*** Bug 442411 has been marked as a duplicate of this bug. ***

I apologize for previously creating a duplicate report of this one. I said that I had issues when running under xrender also, however it was a mistake I no longer have such issues in kwin, expect when utilizing any opengl compositor. Thanks for the attention.

I just hit this effect when using the Eye On Screen effect instead of Window Aperture, so it doesn't seem specific to that effect.

Notably, I also just discovered opening System Settings and switching from one effect to the other makes all the missing windows re-appear.

When we change the options "Show Desktop" to "Minimize all Windows" another problem occurs ( Wayland only ) . Depending on what application is active ( window ) this function works as expected. But when we reshuffle the windows activity , there is a problem.

Description of the Problem : The windows hide, but after clicking again they do not show up. They are still hidden and this function does not work until we manually restore the windows one by one.

Opening the Problem : Launch the Dolphin and Kate applications. Make sure that the windows are visible on your screen. And then remember which window was active. Use the Minimize all Windows function twice via shortcut or applet. If the windows are still active the second time, change the windows activity to the other application and use the function twice again.

PS. I am not quite sure if the function error itself occurs only when using it , or for other reasons. However, if the problem does not appear, please use more applications, and change their activity, surely it will appear.

*** Bug 442649 has been marked as a duplicate of this bug. ***

I don't know if something recent fixed this, but I'm no longer able to reproduce the issue with today's git master. I realize it's been a few days since it happened to me, whereas before I was hitting it at least once a day, sometimes more often.

The bug still exists in 
KDE Plasma Version: 5.22.5
KDE Frameworks Version: 5.85.0

Created attachment 142167 [details]
qjsengine bug reproducer

I found a reliable way to reproduce the issue in QJSEngine. Key points:

* several engines need to work with the same qobject
* if garbage collector runs at some specific point, it seems like qobject wrappers will be destroyed

if you run the demo, you're going to see

js: customProp2 was stripped away for EffectWindow(0x55d553dc1d50)
js: customProp2 was stripped away for EffectWindow(0x55d553dc1b30)
js: customProp2 was stripped away for EffectWindow(0x55d553dc1f70)

ideally, the demo should not print anything.

I think I found the culprit of this bug. The garbage collector seems to prefer to keep "null or undefined" qobject wrappers in MemoryManager::sweep() https://code.woboq.org/qt5/qtdeclarative/src/qml/memory/qv4mm.cpp.html#983

    if (MultiplyWrappedQObjectMap *multiplyWrappedQObjects = engine->m_multiplyWrappedQObjects) {
        for (MultiplyWrappedQObjectMap::Iterator it = multiplyWrappedQObjects->begin(); it != multiplyWrappedQObjects->end();) {
            if (!it.value().isNullOrUndefined())
                it = multiplyWrappedQObjects->erase(it);
            else
                ++it;
        }
    }

That looks very sketchy. If `!it.value().isNullOrUndefined()` is changed to `it.value().isNullOrUndefined()`, tainted qobject wrappers don't get destroyed.

Vlad's patch (https://codereview.qt-project.org/c/qt/qtdeclarative/+/376014) has been merged, so this is officially fixed in Qt. We'll see about backporting the fix to the KDE patch collection for Qt 5.15.

Qt backport request: https://invent.kde.org/qt/qt/qtdeclarative/-/merge_requests/8

*** Bug 443633 has been marked as a duplicate of this bug. ***

*** Bug 444610 has been marked as a duplicate of this bug. ***