the opposite handedness if the desired handedness is missing.
For example, if Alt_R is unmapped as it often is on keyboards
with Mode_switch, the client will use Alt_L if it is mapped
when told to synthesize Alt_R. This should fix handling of
AltGr sent from a win32 server. AltGr is literally just
Control_L and Alt_R and, previously, an X11 client without
Alt_R mapped would only simulate Control_L. This does not
address the fact that the user may really just want the Alt
modifier; there are already hueristics to attempt to figure
that out.
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