Use qEnvironmentVariableIsEmpty?

Are you sure so many people? :P

Use new connect syntax.

Something like

QVariantMap button = {
{ StringLiteral("action"), actionList.at(i)},
{QStringLiteral("label"), actionList.at(i + 1)}
};

would be more readable?

m.setArguments({ QString::number(id) });

QFileInfo::exists supposedly is faster

Note that isServiceRegistered is a blocking DBus call

Also, is the null check for interface required?

Do that in the constructor of NotifyByFlatpakPrivate above

Don't iterate values() (creates a temporary list just to iterate it) since foreach already does that anyways

emit?

QVariantList

Can't you do this in one go? You first create a temporary list of actions and then you create a QVariantMap from them.

Also, please use reserve() if you already know in advance how many items you're going to add to a container.

Maybe instead of querying for dbus things we could figure out a what QPlatformTheme is in use somehow?

Not sure how that would help. It would help maybe to detect whether we are in sandbox if we check whether the platform plugin is "flatpak", but with this check I want to make notifications work at least somehow, even in case we are in sandbox and the portal service is not available from some reason. If you use NotifyByPopup plugin in the sandbox then it will show the ugly notification on top of the screen instead so it's at least some notification. In case you would like to avoid displaying this ugly notification and rely on portals only then we can ignore this check and use NotifyByFlatpak regardless availability of the portal service.

It's a modified "NotifyByPopup" plugin and I didn't change the copyright except adding myself.

These could be a local variable, it's not being used outside for figuring out the backend.