How to map a keybinding to a command which includes <C-U> (scroll-up) in my .vimrc?

Question

I want to map a keybinding to a command which includes <C-U> not for clearing the command line but for scrolling the page up. Executing a command like this in my vim session works totally fine:

:exe "normal \<C-U>"

But when I try to map something to the exact same command in my .vimrc, it doesn't work:

" My .vimrc
nnoremap <leader>u :exe "normal \<C-U>"<CR>

Now when I do <leader>u it leaves :" in my status bar which indicates <C-U> is clearing the line instead of scrolling the page. What is the problem?

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Edit: I oversimplified the keybinding that I'm actually using to only point to what causes the problem and avoid confusion. I am using <C-U> in the context of a much more complicated keybinding but I need it to mean "scroll-up" instead of "clear the line." I'm using this binding nnoremap <leader>wu :<C-U>call win_execute(win_getid(winnr('#')), ":exe 'normal \<C-U>'")<CR> to scroll another window without switching directly to it. I already used it with <C-D> for scrolling down and it works perfectly fine but this one doesn't work because of the other meaning that <C-U> has in command mode.

Answer 1

Alright, so: I am having a remarkably hard time finding the section of the documentation that discusses how the right-hand-side of a mapping is interpreted (i.e., how the keycodes are read during :map and then later used upon execution). What I did discover with some trial-and-error, though, is this:

1. nnoremap ... :execute "normal! <C-u>"<CR> has a literal <C-u> already in it, so that's why that fails. This happens even with the backslash, I think because nnoremap processes the <C-u> before execute/normal/double-quotes ever get to it.
2. To work around that, we need to escape the < using <lt>. However, then we have the mapping running :execute "normal! <C-u>" which already doesn't work.
3. To get around that, we need to put a backslash back into the string; for some reason, I couldn't make \\ work, even though that is supposed too—I may have needed to double (\\\\), but I didn't want to do that. So, I used <Bslash>.

The final mapping is

nnoremap ... :execute "normal! <Bslash><lt>C-u>"<CR>

Phew 😅

P.S. I was able to work through this by reading the output of :map {rhs} and realizing what characters had been pre-processed and what hadn't, as well as actually running the mapping to see what had happened.

P.P.S. I still don't think these shenanigans should be necessary for win_execute; it should be sufficient to pass the normal! command directly (with some pre-escaping if it's in a mapping). For example,

call win_execute(win_getid(winnr()), "normal! \<C-u>")

worked for me. You would probably need to use <Bslash> and <lt>C-u> in the mapping version.