One day, I noticed that some of my mappings had stopped working without any visible reasons. Exactly as Soham did.
In short, it seems that our lovely Vim has a very weird side effect. In my case, adding the » symbol to the string of the langmap option incomprehensibly ruins some of the mappings for other keys and symbols. Wherein, the output of the :map command tells that all must be OK. Yes, in some cases, such as Shift-Tab, the terminal does not pass over the signal, but it’s not the case we’re talking about.
If anyone, as Soham or me, is having the trouble, try to start your investigation from the langmap option. And don’t think, please, that there is any visible connection between the symbols in your langmap and the symbols and keys that stopped working.
Knowing about the terminal behaviour, in order to have more “handles” for Vim, I’ve remapped some of the keyboard mappings via the xkb technology. In particular, I have the Escape signal on the Tab key. On the fifth level of that key (xkb terminology) I have the ↻ symbol which is mapped in Vim to the Ctrl-R command. Then, I have the ” symbol on the x key (on the third level) when I‘m working with a Latin set, and the » symbol on the same key when I‘m working with a Cyrillic set.
So, my langmap is:
The » symbol in the last string (actually located at the x key) ruins the mapping for the ↻ symbol on the Tab key. Yes! It’s very interesting! So, deleting the mapping for the » symbol puts Vim to the right condition.