Request: (XIM) Compose sequences using modifier key releases

[Andreas Wettstein]

Dear all,

I would like to use modifiers keys (such as Control_L or Shift_R) as
dead keys in compose sequences.

Currently, only non-modifiers can be part of a compose sequence, and
they are matched against the defined compose sequences at the time the
key is pressed.  I want to extend this to modifier keys, but in contrast
to non-modifiers, they should be matched to the defined compose
sequences when the key is released, and only when no other key has been
pressed since the modifier had been pressed.

When a compose sequence is ongoing, but a non-matched non-modifier key
is pressed, the compose sequence is aborted.  In contrast, for backward
compatibility, releasing a non-matched modifier during an ongoing
compose sequence should have no effect, rather than aborting the compose
sequence.