The basic problem is this: Thread A has a window W1 that is it's focus
window and the system-global foreground window. At some point thread A
stops processing messages. After that, thread B creates a window W2
and makes it the foreground window. Thread B later on makes W1 (from
Thread A) the foreground window again. After restoring W1 as the
foreground window, Thread A processes window messages again. Two
WM_WINE_SETACTIVEWINDOW messages are in the queue, one for losing the
foreground thread propery and one for restoring it. The first one will
generates a WM_ACTIVATEAPP(0) message, which causes D3D to minimize
the game window.

The included test shows that Windows doesn't deliver any
WM_ACTIVATEAPP messages if the thread stopped being the foreground
thread and re-gained that property between two message processing
calls. It isn't implemented with a plain WM_ACTIVATEAPP filter, the
manually injected message in the test still gets through.
Signed-off-by: Stefan Dösinger <stefan@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>