This is a significant shift in how input events are perceived. The common
approach was to treat a core event as a different entity than the XI event.
This could result in the XI event being delivered to a different client than
the core event. This doesn't work nicely if they come from the same device.
Instead, we treat an input event as a single event, that is delivered through
two separate APIs. So when delivering an event, we first try the XI event,
then the core event. If the window want's neither, we go to the parent and
repeat. Once either core or XI has been delivered, the processing stops.
Important: Different to the previous method, if a client registers for core
button events, the parent window will not get XI events. This should only
cause problems when you're mixing core and XI events, so don't do that!
Generic events don't fit into this yet, they cause me headaches.
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