Blocking echo messages on startup result in blank screen

Steps to reproduce

With the latest prerelease, I run the following command (from a draft default diff tool config similar to https://github.com/jj-vcs/jj/pull/6205, sorry for extraneous quotes):

Update: I changed this line to remove irrelevant backticks. They mess up the result in bash or zsh; I didn't notice because I use fish.

"gvim" "-f" "-d" "-u" "/dev/null" mkdocs-offline.yml mkdocs.yml "-c" "echohl WarningMsg" "-c" "echomsg \"Warning: using plain vimdiff as a diff editor is not recommended. See https://github.com/jj-vcs/jj/wiki/Vim for alternatives.\"" "-c" "echohl None"

(It doesn't matter which two files you'd be comparing, or if they even exist)

This results in a confusing blank screen instead of the expected message:

After pressing Enter, everything works.

Expected behaviour

In plain vim, I get:

"mkdocs-offline.yml" 19 lines, 724 bytes
"mkdocs.yml" 154 lines, 4992 bytes
Warning: using plain `vimdiff` as a diff editor is not recommended. See https://github.com/jj-vcs/jj/wiki/Vim for alternatives.
Press ENTER or type command to continue

Then, vim loads after I press ENTER.

Version of Vim and architecture

1-1251

Environment

VIM - Vi IMproved 9.1 (2024 Jan 02, compiled Mar 28 2025 09:26:05)
macOS version - arm64
Included patches: 1-1251
Compiled by GitHub Actions

How MacVim was installed

Downloaded from GitHub

Logs and stack traces

Vim configuration where issue is reproducable

No response

Issue has been tested with given configuration

• by running MacVim.app from GUI macOS interface
• by running vim/gvim/etc installed by MacVim
• by running other versions of vim (e.g. /usr/bin/vim)

Issue has been tested with no configuration

• by running mvim --clean (or gvim, supplied by MacVim distribution)
• by running vim --clean (in terminal, supplied by MacVim distribution)
• by running vim --clean (in terminal, other suppliers, e.g. /usr/bin/vim)

Other conditions

• The both Homebrew packages "vim" and "macvim" are installed

Comienza reproduciendo el comando con mvim --clean y compara la pantalla de inicio en blanco con la visualización de mensajes de plain vim. Rastrea cómo MacVim gestiona los mensajes de echo del inicio y el lanzamiento del editor de diferencias, y verifica después que la advertencia aparece antes de que el editor pueda utilizarse, sin requerir una pulsación adicional de Enter.