fix(api): buffer updates in quickfix buffer #31105

Problem: Buffer events (specifically on_bytes callbacks) weren't triggered when the
quickfix list was modified, preventing buffer change notifications.

Solution: Add code to send both bytes and lines change notifications after
quickfix buffer updates to properly trigger all attached callbacks.

src/nvim/quickfix.c

@@ -16,6 +16,7 @@
 #include "nvim/autocmd_defs.h"
 #include "nvim/buffer.h"
 #include "nvim/buffer_defs.h"
+#include "nvim/change.h"
 #include "nvim/charset.h"
 #include "nvim/cursor.h"
 #include "nvim/drawscreen.h"
@@ -31,6 +32,7 @@
 #include "nvim/ex_eval.h"
 #include "nvim/ex_eval_defs.h"
 #include "nvim/ex_getln.h"
+#include "nvim/extmark.h"
 #include "nvim/fileio.h"
 #include "nvim/fold.h"
 #include "nvim/garray.h"
@@ -50,6 +52,7 @@
 #include "nvim/message.h"
 #include "nvim/move.h"
 #include "nvim/normal.h"
+#include "nvim/ops.h"
 #include "nvim/option.h"
 #include "nvim/option_defs.h"
 #include "nvim/option_vars.h"
@@ -4149,6 +4152,8 @@ static void qf_update_buffer(qf_info_T *qi, qfline_T *old_last)
   }
 
   linenr_T old_line_count = buf->b_ml.ml_line_count;
+  colnr_T old_endcol = ml_get_buf_len(buf, old_line_count);
+  bcount_t old_bytecount = get_region_bytecount(buf, 1, old_line_count, 0, old_endcol);
   int qf_winid = 0;
 
   win_T *win;
@@ -4182,7 +4187,25 @@ static void qf_update_buffer(qf_info_T *qi, qfline_T *old_last)
   qf_update_win_titlevar(qi);
 
   qf_fill_buffer(qf_get_curlist(qi), buf, old_last, qf_winid);
-  buf_inc_changedtick(buf);
+
+  linenr_T new_line_count = buf->b_ml.ml_line_count;
+  colnr_T new_endcol = ml_get_buf_len(buf, new_line_count);
+  bcount_t new_byte_count;
+
+  linenr_T delta = new_line_count - old_line_count;
+  if (old_last == NULL) {
+    new_byte_count = get_region_bytecount(buf, 1, new_line_count, 0, new_endcol);
+    extmark_splice(buf, 0, 0, old_line_count - 1, 0, old_bytecount, new_line_count - 1, new_endcol,
+                   new_byte_count, kExtmarkNoUndo);
+    changed_lines(buf, 1, 0, old_line_count > 0 ? old_line_count + 1 : 1, delta, true);
+  } else {
+    linenr_T start_lnum = old_line_count + 1;
+    new_byte_count = get_region_bytecount(buf, start_lnum, new_line_count, 0, new_endcol);
+    extmark_splice(buf, old_line_count - 1, old_endcol, 0, 0, 0, delta, new_endcol, new_byte_count,
+                   kExtmarkNoUndo);
+    changed_lines(buf, start_lnum, 0, new_line_count + 1, delta + 1, true);
+  }
+  buf->b_changed = false;
 
   if (old_last == NULL) {
     qf_win_pos_update(qi, 0);

test/functional/lua/buffer_updates_spec.lua

@@ -420,6 +420,35 @@ describe('lua buffer event callbacks: on_lines', function()
     feed('i<Tab>')
     eq({ '\ta' }, exec_lua('return _G.res[#_G.res]'))
   end)
+
+  it('quickfix buffer send change', function()
+    command('copen')
+    exec_lua(function()
+      vim.api.nvim_buf_attach(vim.api.nvim_get_current_buf(), false, {
+        on_lines = function(...)
+          vim.g.qf_on_lines = { ... }
+        end,
+        on_bytes = function(...)
+          vim.g.qf_on_bytes = { ... }
+        end,
+      })
+    end)
+    command('caddexpr "foo"')
+    eq({ 'bytes', 2, 2, 0, 0, 0, 0, 0, 0, 0, 6, 6 }, api.nvim_get_var('qf_on_bytes'))
+    eq({ 'lines', 2, 3, 0, 1, 1, 1 }, api.nvim_get_var('qf_on_lines'))
+
+    command('caddexpr "bar"')
+    eq({ 'bytes', 2, 3, 0, 6, 6, 0, 0, 0, 1, 6, 6 }, api.nvim_get_var('qf_on_bytes'))
+    eq({ 'lines', 2, 4, 1, 2, 4, 0 }, api.nvim_get_var('qf_on_lines'))
+
+    command('caddexpr ["line1", "line2", "line3"]')
+    eq({ 'bytes', 2, 4, 1, 6, 13, 0, 0, 0, 3, 8, 26 }, api.nvim_get_var('qf_on_bytes'))
+    eq({ 'lines', 2, 5, 2, 5, 9, 0 }, api.nvim_get_var('qf_on_lines'))
+
+    command('cexpr "replace"')
+    eq({ 'bytes', 2, 5, 0, 0, 0, 4, 0, 40, 0, 10, 10 }, api.nvim_get_var('qf_on_bytes'))
+    eq({ 'lines', 2, 6, 0, 5, 1, 42 }, api.nvim_get_var('qf_on_lines'))
+  end)
 end)
 
 describe('lua: nvim_buf_attach on_bytes', function()