fix(textformat): don't swallow "space" with auto-formatting enabled (#38181)

vim-patch:9.2.0124: auto-format may swallow white space

Problem:  With auto paragraph formatting enabled, when a user makes an
          attempt to add a new word before the end of a line and with
          the following space bringing the line width over 'textwidth',
          the space ends up just getting swallowed by the editor.
Solution: Detect such a constellation and do not auto-format in that
          case (Daniel Müller).

closes: vim/vim#19593

24fd698045

src/nvim/textformat.c

@@ -646,6 +646,22 @@ void auto_format(bool trailblank, bool prev_line)
     curwin->w_cursor = pos;
   }
 
+  // Also skip formatting when the user just typed whitespace in the
+  // middle of the line.  Reformatting would join all paragraph lines and
+  // re-wrap, consuming the space at the line break point via
+  // OPENLINE_DELSPACES.  By deferring, the next non-whitespace character
+  // will be inserted adjacent to the space, keeping it protected from
+  // being consumed at a line break.  auto_format() will then reformat
+  // properly on the next keystroke.
+  if (*old != NUL && !trailblank && !wasatend && pos.col > 0
+      && (State & MODE_INSERT)) {
+    char *line = get_cursor_line_ptr();
+    if (WHITECHAR(line[pos.col - 1])) {
+      curwin->w_cursor = pos;
+      return;
+    }
+  }
+
   // With the 'c' flag in 'formatoptions' and 't' missing: only format
   // comments.
   if (has_format_option(FO_WRAP_COMS) && !has_format_option(FO_WRAP)

test/old/testdir/test_textformat.vim

@@ -1158,6 +1158,32 @@ func Test_fo_a_w()
   %bw!
 endfunc
 
+" Test that auto-format ('a' flag) preserves spaces typed in the middle of a line
+func Test_fo_a_midline_space()
+  new
+  let lines = [
+    \ 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa it but',
+    \ 'Lorem Ipsum is simply dummy text of the printing and typesetting dasddd',
+    \ 'industry.',
+    \ ]
+  call setline(1, lines)
+  set fo=ta tw=70
+
+  " Prevent INPUT_BUFLEN batching so auto_format runs between keystrokes
+  autocmd InsertCharPre * " nothing
+
+  " Position at 't' of 'it' (col 68) and type space then Z
+  call cursor(1, 68)
+  call feedkeys("a Z\<Esc>", 'xt')
+
+  " The space between 'it' and 'Z' must be preserved
+  call assert_match('it Z', getline(1))
+
+  autocmd! InsertCharPre
+  set fo& tw&
+  bw!
+endfunc
+
 " Test for formatting lines using gq in visual mode
 func Test_visual_gq_format()
   new