About IWMPWindowMessageSink (deprecated)
This page documents a feature that may be unavailable in future versions of Windows Media Player and the Windows Media Player SDK.
When the plug-in is active without a window hosted in the Player user interface, such as when the Player is rendering Windows Media video or when in windowless mode, it requires a mechanism to receive window messages like mouse and keyboard events. Windows Media Player sends these messages to the plug-in by calling IWMPWindowMessageSink::OnWindowMessage.
Windows Media Player queries all active rendering plug-ins to determine whether they support this interface and automatically forwards window messages to each plug-in that does.