Problem:
`vim.on_key()` callbacks are invoked for keys that `getchar()` consumes,
which never reach the main loop, and there is no way to tell such a key
apart. With ui2 the `on_key` handler activated after interactive
cmdline, handles it as `pager_char` (thus enters the ui2 pager).
Solution:
Skip `vim.on_key()` callbacks during `getchar()`/`getcharstr()`.
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