[vim/vim] ins-completion menu is not align with start positon (Issue #20868)

mao-yining created an issue (vim/vim#20868)

For

```vim
vim9script
def g:Path(findstart: bool, base: string): any
	if findstart
		var pos  = col('.') - 2
		var orig = pos
		var line = getline('.')

		while pos > 0 && line[pos] =~ '\f'
			--pos
		endwhile

		if pos == 0 && line[pos] =~ '\f'
			return 0
		elseif pos == orig
			return -2
		else
			return pos + 1
		endif
	endif
	try
		return base->getcompletion('file')
			->mapnew((_, v) => ({word: v->trim('/\', 2), abbr: v, kind: 'p'}))
	catch
		return v:none
	endtry
enddef
set complete=FPath
set autocomplete
```

menu is placed in different place

Current behaviour:

<img width="792" height="218" alt="Image" src="https://github.com/user-attachments/assets/4f933be5-4d0a-4b13-8cb0-816245168668" />

Expect like: `<C-X><C-F>`

<img width="506" height="210" alt="Image" src="https://github.com/user-attachments/assets/1045c738-1d0f-4387-bf07-fdb243241bb0" />

Find while: #20392.

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