fix(prompt): handle multi-element lists in prompt_appendbuf #39550

Problem:
When using prompt_appendbuf with multi-element list,
the first item is concated and rest replace the prompt instead of
inserting the lines before the prompt.

Solution:
Concat first element with replace_buf and insert the rest of the list
with set_buffer_lines.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>
(cherry picked from commit a977e1077b)

src/nvim/eval/buffer.c

@@ -291,6 +291,8 @@ void f_prompt_appendbuf(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 
   linenr_T lnum = MAX(0, buf->b_prompt_start.mark.lnum - 1);
   typval_T *lines = &argvars[1];
+  bool did_concat = false;
+
   if (!buf->b_prompt_append_new_line) {
     // Since we are not creating a new line we need to append input to current line
     const char *text = (lnum > 0) ? (const char *)ml_get_buf(buf, lnum) : "";
@@ -303,6 +305,7 @@ void f_prompt_appendbuf(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
         tv_clear(&li->li_tv);
         li->li_tv.v_type = VAR_STRING;
         li->li_tv.vval.v_string = new_str;
+        did_concat = true;
       }
     } else if (lines->v_type == VAR_STRING) {
       const char *str = tv_get_string(lines);
@@ -314,7 +317,20 @@ void f_prompt_appendbuf(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   }
 
   if (did_emsg == did_emsg_before) {
-    set_buffer_lines(buf, lnum, buf->b_prompt_append_new_line, lines, rettv);
+    if (did_concat && tv_list_len(lines->vval.v_list) > 1) {
+      // Multi-element list with concat: first element (already concatenated)
+      // replaces the line at lnum, remaining elements are inserted after.
+      list_T *l = lines->vval.v_list;
+      listitem_T *li = tv_list_first(l);
+      set_buffer_lines(buf, lnum, false, &li->li_tv, rettv);
+
+      if (rettv->vval.v_number == 0) {
+        tv_list_item_remove(l, li);
+        set_buffer_lines(buf, lnum, true, lines, rettv);
+      }
+    } else {
+      set_buffer_lines(buf, lnum, buf->b_prompt_append_new_line, lines, rettv);
+    }
   }
 
   if (rettv->vval.v_number == 0) {

test/functional/legacy/prompt_buffer_spec.lua

@@ -1073,4 +1073,56 @@ describe('prompt buffer', function()
     fn('prompt_setprompt', unloaded_buf, 'hello unloaded! > ')
     eq('hello unloaded! > ', fn('prompt_getprompt', unloaded_buf))
   end)
+
+  it('prompt_appendbuf with multi-element list and singleline prompt', function()
+    command('new')
+    local buf = api.nvim_get_current_buf()
+    api.nvim_set_option_value('buftype', 'prompt', { buf = buf })
+    fn('prompt_setprompt', buf, 'cmd: ')
+
+    -- Single element list: appends before prompt
+    fn('prompt_appendbuf', buf, { 'line1' })
+    eq({ 'line1', 'cmd: ' }, api.nvim_buf_get_lines(buf, 0, -1, false))
+
+    -- Multi-element list: first element appended, rest inserted as new lines before prompt
+    fn('prompt_appendbuf', buf, { '-append', 'line2' })
+    eq({ 'line1-append', 'line2', 'cmd: ' }, api.nvim_buf_get_lines(buf, 0, -1, false))
+
+    -- Multi-element list after multi-element list
+    fn('prompt_appendbuf', buf, { '', 'line3', 'line4', 'line5', 'line6' })
+    eq(
+      { 'line1-append', 'line2', 'line3', 'line4', 'line5', 'line6', 'cmd: ' },
+      api.nvim_buf_get_lines(buf, 0, -1, false)
+    )
+  end)
+
+  it('prompt_appendbuf with multi-element list and multiline prompt', function()
+    command('new')
+    local buf = api.nvim_get_current_buf()
+    api.nvim_set_option_value('buftype', 'prompt', { buf = buf })
+    fn('prompt_setprompt', buf, 'cmd: ')
+    source('startinsert')
+
+    -- User types multiline input in the prompt
+    feed('input1<s-cr>input2')
+    eq({ 'cmd: input1', 'input2' }, api.nvim_buf_get_lines(buf, 0, -1, false))
+
+    -- Single element list: appends before prompt
+    fn('prompt_appendbuf', buf, { 'line1' })
+    eq({ 'line1', 'cmd: input1', 'input2' }, api.nvim_buf_get_lines(buf, 0, -1, false))
+
+    -- Multi-element list: first element appended, rest inserted as new lines before prompt
+    fn('prompt_appendbuf', buf, { '-append', 'line2' })
+    eq(
+      { 'line1-append', 'line2', 'cmd: input1', 'input2' },
+      api.nvim_buf_get_lines(buf, 0, -1, false)
+    )
+
+    -- Multi-element list after multi-element list
+    fn('prompt_appendbuf', buf, { '', 'line3', 'line4', 'line5', 'line6' })
+    eq(
+      { 'line1-append', 'line2', 'line3', 'line4', 'line5', 'line6', 'cmd: input1', 'input2' },
+      api.nvim_buf_get_lines(buf, 0, -1, false)
+    )
+  end)
 end)