What would be your strategy regarding Event::test(const QEvent *event, const char *name)?

There the event name stuff is used. Or could the (event->type() == (QEvent::Type)(QEvent::User + KPARTS_EVENT_MAGIC)); check already be sufficient?

String comparisons are an awful solution for a dynamic_cast-like solution. We were young and inexperienced...
Nowadays we should use QEvent's type() instead.
I suggest to remove eventName() and the Event(const char*) constructor in KF6 and replace it all with event type ids like QEvent does. I would also remove KPARTS_EVENT_MAGIC, the base class should be abstract, it makes not sense to instanciate it as is.
This means

protected:
    explicit Event(int eventType);

and subclasses would do

static const int s_openUrlEventType = 0x0583C101;
OpenUrlEvent::OpenUrlEvent(...)
     : Event(s_openUrlEventType) { ... }

bool OpenUrlEvent::test(const QEvent *event) /* KF6 TODO: const */
{
    return event->type() == s_openUrlEventType;
}

where the type id was generated with the shell command
printf "0x%08X\n" $(shuf -i0-4294967295 -n 1)
(though it will give you a different number if you try) :-)

To be repeated in each subclass.

I think it can be done in KF5 if the test() method does both tests (id and eventName) for now.

But I am still not sure if we even need the Event baseclass then. All the individual events could extend from QEvent and then provide a static test method.

You're right, after removing the string-comparison stuff there's nothing left in KParts::Event, it could be removed completely.

where the type id was generated with the shell command
printf "0x%08X\n" $(shuf -i0-4294967295 -n 1)
(though it will give you a different number if you try) :-)

Is shuf -i1000-65535 -n 1 more correct, because it respects the user-id space?

User = 1000,                            // first user event id
MaxUser = 65535                         // last user event id

https://invent.kde.org/frameworks/kparts/-/merge_requests/57 is the MR