... because their designs are junk resulting in junk APIs. Example: keyboard input. Apparently, you have a message telling you which key was pressed. In fact, you do not, since there is another unrelated to messages API to get the status of register keys. Now, try to make some window believe Ctrl-F1 is pressed when F1 is pressed in any window?
This gets more troublesome with time as Microsoft gets more reasons to pretend it thinks about security. Try to duplicate the Mixer in Windows 7.
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