Unable to imap Ctrl-Shift-U in gvim

Question

I would like to bind Ctrl-Shift-U in gvim so that it enters a template text that I frequently need. For example:

imap <C-S-u> foo

This does not work at all, if I do this, it changes from INSERT mode to "IM INSERT" mode which I have never seen before. So it seems C-S-u is already bound to something, but I cannot figure out how to unbind it. If I say

unmap <C-S-u>

I get E31: No such mapping.

My mapping works just fine with <C-S-l>, but I want <C-S-u> for this. How do I find out what <C-S-u> is bound to and how to unbind it so I can reuse it for my purpose?

Answer 1

This is because you use GTK Vim and GTK implements its own input module (see what IM INSERT means?) which enters "Unicode mode" on Ctrl+Shift+U.

You can disable the GTK input module by launching Gvim with

GTK_IM_MODULE=xim gvim

at which point your desired map will take effect.

For more details, see my answer to Disable transient symbols for compose and dead keys in GTK programs.