recommended `finish-args`/`--socket` settings?

[cbm755]

Since I first packaged by Qt app, even before com.riverbankcomputing.PyQt.BaseApp, I've had:

finish-args:
  - --socket=wayland
  - --socket=x11
  - --share=ipc
  - --device=dri
  - --share=network
  - --filesystem=home

I gather I instead should be using fallback-x11 (b/c of flathub/org.plomgrading.PlomClient#32 and flathub/org.plomgrading.PlomClient#16)

So I changed it to:

finish-args:
  - --socket=wayland
  - --socket=fallback-x11
  - --share=ipc
  - --device=dri
  - --share=network
  - --filesystem=home

But now I get:

flatpak run org.plomgrading.PlomClient 
qt.qpa.xcb: could not connect to display 
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.

I guess I can revert my change but I'm very confused by all this socket stuff. for example, this project's example doesn't mention finish-args: at all (!)
https://github.com/flathub/com.riverbankcomputing.PyQt.BaseApp/blob/branch/5.15-21.08/examples/PyQtApp.yaml

It would be helpful to have an example of "best practice" into there.

[cbm755]

It seems I have QT_QPA_PLATFORM=xcb in my .bashrc and (very surprisingly to me) that effects the container!

But I still find those options confusing so if there is helpful best practice examples we could show in the PyQtApp.yaml that would likely help new users.

[tinywrkb]

It sounds like you're a Gnome user. It's a known Qt5 behavior to force XCB QPA platform when Gnome desktop is detected, and specifically when XDG_CURRENT_DESKTOP contains the entry GNOME.
Allegedly this was due to Qt bugs, and this was finally dropped in Qt 6.3.

[tinywrkb]

And here's the offending line.

The optimal solution is to move forward to Qt6.
I'm not sure how wise it would be to add some kind of logic (e.g. with a wrapper) to force Wayland QPA platform when running the app in Gnome environment considering that Qt's developers determine that this combination of Gnome and the Qt5's Wayland QPA platform is not ready.

[cbm755]

Thanks! I agree that sounds largely like "Someone Else's Problem" and not something to hack around in this project.

Would it be useful to advise users of com.riverbankcomputing.PyQt.BaseApp of this situation? Perhaps something like:

1. If you are using base-version: 5.15-21.08 (Qt5-based), you may want to use:

   finish-args:
     - --socket=wayland
     - --socket=x11
   

2. If your base-version starts with 6, consider instead using:

   finish-args:
     - --socket=wayland
     - --socket=fallback-x11
   

   (see Issue recommended finish-args/--socket settings? #111, "could not load Qt platform plugin" org.musescore.MuseScore#52, should I be using --socket=fallback-x11? org.plomgrading.PlomClient#16, Tracker for applications using --socket=x11 and --socket=wayland flathub#1452)

[hfiguiere]

It sounds like you're a Gnome user. It's a known Qt5 behavior to force XCB QPA platform when Gnome desktop is detected, and specifically when XDG_CURRENT_DESKTOP contains the entry GNOME. Allegedly this was due to Qt bugs, and this was finally dropped in Qt 6.3.

This is actually not true in flatpak as the following patch is applied:

https://invent.kde.org/packaging/flatpak-kde-runtime/-/blob/qt5.15lts-23.08/patch/qtbase-use-wayland-on-gnome.patch?ref_type=heads