[vim/vim] message: preserve empty lines when redrawing message scrollback (PR #21444)

[赤坂理子]

Fixes #21315

Problem

• Reproducible on upstream commit 6d862f5 with :echo join(range(1, 1000), "\n\n") followed by f then b at the more prompt
• Patch 9.0.0595 discarded standalone newline chunks, causing empty separator lines to vanish when redrawing scrollback

Solution

• Store newline chunks for scrollback again in src/message.c
• Update msg_sb_eol to avoid marking a trailing newline-only chunk as finished so the older extra-newline bug does not return
• Add regression coverage in src/testdir/test_messages.vim with three new screendumps exercising single, double, and triple newline separators, comparing the initial page to f/b and PageDown/PageUp redraws
• The new regression test fails on the unpatched Linux binary and passes after this fix

Validation

• make -C src test_messages TEST_FILTER=Test_message_scrollback_empty_lines passed
• make -C src test_messages test_cmdline test_vim9_cmd passed on macOS and Ubuntu Linux with platform/feature skips
• make -C src/testdir codestyle passed on macOS and Ubuntu Linux
• Same three test suites passed on Linux Clang with AddressSanitizer, UndefinedBehaviorSanitizer, and leak detection
• Linux --with-features=tiny build and make -C src testtiny passed
• git diff --cached --check passed
• Existing Test_echo_verbose_system passed on Linux, skipped by the existing Mac guard on macOS
• Resolved local prerequisites including PTY allocation, non-root user execution, and Clang sanitizer runtime
• GitHub CI has not run yet

---

You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/21444

Commit Summary

• 8fe0460 fix: preserve empty lines in message scrollback

File Changes

(5 files)

• M src/message.c (10)
• A src/testdir/dumps/Test_message_scrollback_1.dump (10)
• A src/testdir/dumps/Test_message_scrollback_2.dump (10)
• A src/testdir/dumps/Test_message_scrollback_3.dump (10)
• M src/testdir/test_messages.vim (29)

Patch Links:

• https://github.com/vim/vim/pull/21444.patch
• https://github.com/vim/vim/pull/21444.diff

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[h_east]

h-east left a comment (vim/vim#21444)

The msg_sb_eol() change is not covered by the new test

The comment in the new test says the single newline case checks that
no extra empty line is introduced. I reverted only the msg_sb_eol()
hunk and the new test still passes, newline_count 1 included. What
fails is the existing Test_echo_verbose_system(). So the comment
claims something the loop does not check. Please drop it, or point
to Test_echo_verbose_system() instead.

g< also changes

With this PR, g< (and G at the more prompt) shows the trailing
empty lines of a message, as they were on the screen:

:echo "a\n"
" press Enter, then g<

master shows "a" and the prompt right below it; this PR shows the
empty line between them, like the screen did. That is right, but it
is a visible change that the description does not mention and no test
covers. A g< step in the new test would pin it down.

Comment in msg_puts_display()

"including empty lines" refers to the condition that was removed.
Without it the line reads fine:

    // Store the text for scroll back.

Commit message

The subject should state the problem, and the body needs the
Problem:/Solution: lines. A pull request template was added about a
week ago and covers this, including the AI disclosure:
6918007
Could you fill it in?

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[赤坂理子]

akasakariko left a comment (vim/vim#21444)

I'll address these

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