Allows such IMEs that process keyboard input to do so. For instance, fixes backspace and escape for handwriting methods.
Unfortunately there doesn't seem to be a way to detect the activation of non-keyboard IMEs, so this needs to be checked per-keystroke. Some rough benchmarking shows that it adds about 15 microseconds of overhead, which is somewhat dwarfed by the ~50 microseconds for the dispatch to the main thread, which was always already happening.