A customer had a program that supported the mouse wheel, and they found that some laptops have trackpad enhancement software which supports a gesture for turning the mouse wheel. When the enhancement software recognizes the gesture, it sends a WM_MOUSE­WHEEL message to the foreground window.. This causes problems for the program because there are various things that are not allowed when processing an inbound sent message. Fortunately, they can detect whether they are in this situation, but it’s not clear how they can recover from it. “We have been brainstorming and thinking that if we see this problem, we will use Post­Message to put the message ‘back’ into our queue.”

The trackpad software is supposed to be using Send­Input so that the wheel message orders correctly with the other messages in your input queue. The Send­Message delivers the message immediately (or as immediately as possible), and messages posted with the Post­Message function are processed ahead of input, both of which are wrong when you are trying to simulate input. Even your attempt to delay processing by calling Post­Message won’t help because posted messages are processed ahead of input.

Here’s a diagram which is inaccurate but may help to get the point across:

Basically, the enhancement software already screwed you with respect to message ordering. You won’t be able to make a perfect recovery; all you can do is try to make the best of a bad situation.

One idea is to use the Reply­Message function in your message handler. The Reply­Message function says, “Hey, like, act like I returned from this message for the purpose of inter-thread Send­Message bookkeeping, such as unblocking the sender, but let me keep running anyway.” This may be enough to get the parts of the system that normally say “No, you can’t do that from inside a sent message” to realize “Oh, wait, the synchronous part is over. Carry on.”

If that doesn’t work, then you can use the Send­Input message to generate a wheel message back into the input queue. The wheel input will be a bit late (by the amount of time it took your window procedure to receive the message), but it’ll probably be close enough to correct that most people won’t notice. There’s most likely already a lag in the gesture recognition in the enhancement software, so a little more lag probably isn’t the end of the world.