Among other things, this fixes Syberia 2. That game shows, hides, and then
shows its window. Hiding it caused a WINDOW_LOST_FOCUS event to be queued.
By the time it was processed, the window was the foreground window again.
In response to being told it had lost focus, the game minimized its window.
Hiding the window should have prevented or discarded the WINDOW_LOST_FOCUS
event since the change was driven from Wine and the Win32 foreground/active
window state would already be correct. In addition, when the program
re-showed its window and made it foreground, that should have discarded the
event as being out of date. Now they do.
The main dispatch queue is a serial queue and is a shared resource. If we
submit a long-running task to it, then no other tasks, including those submitted
by the system frameworks, can run until it completes.
Some events get queued for all GUI-connected threads but are only processed
by the first to dequeue them. Other threads which tend their event queue
discard such already-processed events. However, some threads may be
connected to the GUI but never tend their event queue. To prevent such
threads from accumulating zombie events, the zombies are cleared each time a
new event is queued.
Some events are application-wide, not specific to a thread. Such an event
needs to be broadcast to all GUI-attached threads because we don't know which
are handling events, but we don't want the event to be processed in each.
Often it should only be processed by the first to pull it from its queue.
