In some cases, WM's will modify the _NET_WM_STATE of our own windows.
Most notably, this can happen when the WM maximizes our window, but
mutter has been known to alter the fullscreen state as well. If we
want to reconfigure our window later, we'll probably have to remove
these states, which means we need to remember that they were set.

Also lock/unlock the module only on creation/destruction of the object.

Also lock/unlock the module only on creation/destruction of the object.

Various internal details about the order and error handling of completion
actions are documented in "Concurrent Programming on Windows" by Joe Duffy.

An instance can only have one completion of each type, trying to add a
second one leads to an exception on Windows.

The instance is marked as long-running even if TpCallbackMayRunLong fails,
a second call will lead to an exception on Windows.

user32/tests: Add some tests to show that WS_CLIPCHILDREN doesn't exclude children from update region.