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Vim horizontal scrollbind skips first scroll if it's the very first interaction

I have a custom spreadsheet viewer in Vim that splits the headers into a top pane and the rest into a bottom pane. I bind horizontal scrolling so that scrolling right keeps the headers above the content.

The problem is that if the _very first thing_ I do is scroll to the right, the header in the top pane doesn't come with it. Subsequent scroll commands line the headers up again. Even a second scroll right. Moving the cursor doesn't line them back up. But if I move the cursor before the first scroll right, the scrollbind works right away.

```vim
" :HeadPager
" Turn vim into a pager with a header row
" Adapted from https://unix.stackexchange.com/a/27840/143088
function! HeadPager()
    set noswapfile
    set nocompatible
    set nowrap
    set scrollopt=hor
    set scrollbind

    " Delete/copy header lines
    silent 1,2delete

    " Split screen with new 1-line buffer (opens at top)
    1new

    " Switch to top split to paste header
    wincmd k
    put!
    goto

    " Switch back to bottom split
    wincmd j

    " Arrows do scrolling instead of moving
    nmap <silent> <Up> 3<c-Y>
    nmap <silent> <Down> 3<c-E>
    nmap <silent> <Left> zH
    nmap <silent> <Right> zL
    nmap <Space> <PageDown>
endfun
command! -nargs=0 HeadPager call HeadPager()
```

I have elided some syntax highlighting, status bar text, and ignoring buffer changes to the file.

```sh
vim the_table.txt -u None -s the_pager.vim -c HeadPager
```

- Why doesn't my first right-scroll bring the top pane along with it?
- Why would it start working _after_ the first scroll?
- ~~How can I lock them from the very beginning?~~
    + Adding a `normal h` after `wincmd j` is enough to be the "first" interaction.