'statusline' %{…} expressions will sometimes lose a leading space

[scy]

Problem

There seems to be some obscure, undocumented logic when displaying the status line that will remove a single space character in some situations.

Steps to reproduce

• nvim --clean foo.txt
• Insert some text to make sure the file is modified.
• :set stl=%f%{&modified?'\ M':''}
• The status line should say foo.txt M
• :set stl=%{expand('%%')}%{&modified?'\ M':''}
• The status line should now say foo.txtM, missing the space before the M.

Expected behavior

The leading space returned by the second expression should not be removed, and certainly not depending on what the first expression is.

Looking at vim/vim#3898, the Vim folks seem to be aware of this since 2017, but it looks to me as if they hesitate changing it, for xkcd 1172 reasons. I'm raising this here, maybe Neovim is willing to break backwards compatibility to make things more consistent.

• In case you're thinking that the %% in the first expression is the problem: :set stl=%{''}%{&modified?'\ M':''} has the status line only contain M, with no leading space either.
• Using %F or %t as the first part will keep the space, while %r or %M will drop it.
• Adding a second space in the second expression (%{&modified?'\ \ M':''}) will result in one being printed.

Neovim version (nvim -v)

NVIM v0.10.0

Vim (not Nvim) behaves the same?

yes, VIM 9.0.1378 (including 1499)

Operating system/version

Debian 12 bookworm

Terminal name/version

kitty 0.26.5

$TERM environment variable

xterm-kitty

Installation

AppImage, but also occurs with Debian's 0.7.2