There's this thing called a message-only window, which is a very misleading name because receiving messages is the thing a window spends all its time doing anyway. It's like calling something a water-only swimming pool or a heat-only oven.
It's also a very misleading name because it doesn't receive all messages. "Wait, the name of this window says that the only thing it can do is receive messages, and now you're saying it can't do even that!"
The point of a message-only window is that it receives only
messages sent or posted specifically to it.
You use it to set up a private channel between the sender
and the window.
After creating a message-only window, you can put messages
in the window's queue by calling PostMessage
and passing that window handle,
or you can send a non-queued message by
calling SendMessage
and passing that window handle.
What makes a message-only window interesting is that it doesn't particpate in broadcast messages.
Many window messages are sent to all top-level windows.
WM_QUERYENDSESSION,
WM_SETTINGCHANGE,
WM_DDE_INITIATE.
and anything sent with
HWND_BROADCAST.
These messages don't reach message-only windows.
Internally, message-only windows are treated as child
windows of a system-managed common parent window called
HWND_MESSAGE.
This system-managed common parent window is permanently
invisible, which results in message-only windows being
permanently invisible.
And that's also how message-only windows are invisible
to enumeration and broadcasts:
Enumeration and broadcasting is done to top-level windows,
but message-only windows are internally treated as child
windows of
HWND_MESSAGE
and therefore are not considered top-level.