sub-window or child with a transient parent?

The file manager Window has to be parented, because pcmanfm-qt runs as a single process.

FYI:
When pcmanfm-qt is used as a file manager, it just fires a dbus call and quits afterwards (explains why the process is short-lived).

That means:
When pcmanfm-qt receives the dbus call (I will send it for debugging) it creates a QMainWindow as child of the QApplication instance. Otherwise the LXQt session would create a mem-leak on quit (and likely also segfault).

(LXQt can probably workaround that by using a smart-pointer instead of parenting the window.)

Actually I was wrong with the assumption of the parented window. The pcmanfm-qt MainWindow is a QMainWindow and unparented!

Presumingly KWin makes the false assumption here that an application can only have one top-level window (parent == nullptr)!

This is independent of window attributes and definitely a KWin bug. LXQt cannot workaround that!

Here's a test application that cleanly reproduces that KWin behaviour:
https://gitlab.com/antis81/kwindesktopwindow

Actually this is rather simple to solve by enhancing the transient strategy. Would you accept a "collaboration" by LXQt on that issue?

FWIW, I cannot reproduce the issue on wayland with the demo running as an xwayland client.

(In reply to Nils Fenner from comment #5)
> Actually this is rather simple to solve by enhancing the transient strategy.
> Would you accept a "collaboration" by LXQt on that issue?
Sure, if it's a kwin bug, we would be happy to merge the fix.

Thanks for the feedbaack Vlad! We will discuss it and propose a fix.

Feel free to ping me in #kde-kwin on OFTC. I'm zzag.

@Vlad From our IRC discussion:

All comes down to `BaseClient::bolongsToSameApplication()` (X11 client does not query this!). By changing the code in https://invent.kde.org/plasma/kwin/-/blob/master/src/layers.cpp#L736 it should work:

```c++
bool X11Client::belongsToDesktop() const
{
    const auto clients = group()->members();

    return std::any_of(clients.constBegin(), clients.constEnd(),
        [this](const AbstractClient *client) {
            if (belongsToSameApplication(client, SameApplicationChecks())) {
                return client->isDesktop();
            }
            return false;
        }
    );
}
```

Since my login doesn't work currently can you do that?

@Vlad Setting the window modality in the demo application has no effect on both (desktop and/or normal) windows. And it makes sense! Had a look at that deprecated Qt::WA_GroupLeader flag. If you search for "groupLeader" in kwin gitlab you will find the code is disabled.

Talking KWin X11Client here:
In addition we have "lxqt-panel" (separate process). Noteworthy that it doesn't set  "Qt::WA_X11NetWmWindowTypeDesktop" flag and instead "Qt::WA_X11NetWmWindowTypeDock".

What we have:
1. pcmanfm -> the only place where the "desktop" flag is set (https://github.com/lxqt/pcmanfm-qt/blob/aa2dc974840521afcd65a2e160641c2eef99ca16/pcmanfm/desktopwindow.cpp#L89).
2. lxqt-panel -> set the "dock" flag (https://github.com/lxqt/lxqt-panel/blob/7c0478ef9c7fc8998f7a7e0c844a8921e038d04d/panel/lxqtpanel.cpp#L167)

KWin filters all "clients" (regardless of the process!) and determines which "belong to the desktop". With wayland it seems to be handled well now by that "KeepAbove" + "belongsToSameApplication" magic.

Actually checking on the "dock" flag when cycling in "X11Client::belongsToDesktop" should work in both Plasma and LXQt (KDE independent) shouldn't it? (Does the Plasma panel set the "dock" attribute? If not, has it another attribute suitable?).