It unfortunately doesn't work as well as XTestFakeKeyEvent() for most uses, as modifiers aren't latched, and it seems that key press and key releases need to happen together to work adequately. All this seems to be because XKB doesn't apply the xtest mappings: /usr/share/X11/xkb/compat/xtest when emitting the key press/releases on behalf of another device.
See also: https://bugzilla.gnome.org/show_bug.cgi?id=670688
Can you provide a more specific example here? The three test cases here latch fine, regardless of whether it's standard XTest or the device-specific function. And in the server code, the device-specific path is largely identical, minus the deviceid handling. Note that the xtest mapping is for the X test suite, not for the XTest extension.
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