Comment # 4 on bug 1059438 from Tim Real

It is both control keys that are hijacked, which makes 
 all manner of trouble for operations involving the control keys.

For example Ctrl + left arrow and Ctrl + right arrow 
 are universally known as text selection operations,
 pretty much since PC day one. It is just... expected behaviour,
 but it's broken because of this compose mapping.

IMO this current mapping is totally wrong.

I have Windows keys on my keyboard, but they don't compose.
Nor would I want them to. Nor would I want /any/ compose functions
 by default, especially those which steal critical keys used 
 for every-day universally accepted standard operations.

IMO it is pretty much guaranteed that a new user would be put off 
 by this behaviour. Look how long it took /me/ to deal with it,
 and I'm a coder...

Thanks.