From 47e97c1ad64342dd89a4cd5209e0a5b5fa3ea828 Mon Sep 17 00:00:00 2001
From: zeertzjq <zeertzjq@outlook.com>
Date: Wed, 22 Jul 2026 22:03:05 +0800
Subject: [PATCH] vim-patch:9.2.0827: :startinsert enters Insert mode in a
 non-modifiable buffer (#40906)

Problem:  ":startinsert" enters Insert mode in a buffer where
          'modifiable' is off, the error only appears when a character
          is typed.  Typing "i" gives the error right away (Barrett Ruth)
Solution: Give the error when the buffer is not modifiable, like "i"
          does.  Keep ignoring the command in a terminal window, where
          ":startinsert" is documented to be ineffective, and keep
          accepting it when 'insertmode' is set, like "i" does
          (Hirohito Higashi).

fixes:  vim/vim#20804
closes: vim/vim#20806

https://github.com/vim/vim/commit/6ab1976e49135c0d03b06e3cddd26dc86edff085

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>
---
 src/nvim/ex_docmd.c            |  4 +++
 test/old/testdir/test_edit.vim | 53 ++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index f34551a299..d8f4983beb 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -7380,6 +7380,10 @@ static void ex_normal(exarg_T *eap)
 /// ":startinsert", ":startreplace" and ":startgreplace"
 static void ex_startinsert(exarg_T *eap)
 {
+  if (!curbuf->b_p_ma && !curbuf->terminal) {
+    emsg(_(e_modifiable));
+    return;
+  }
   if (eap->forceit) {
     // cursor line can be zero on startup
     if (!curwin->w_cursor.lnum) {
diff --git a/test/old/testdir/test_edit.vim b/test/old/testdir/test_edit.vim
index cb78086bcb..e886089d26 100644
--- a/test/old/testdir/test_edit.vim
+++ b/test/old/testdir/test_edit.vim
@@ -1780,10 +1780,50 @@ func Test_edit_startinsert()
   call feedkeys(":startinsert!\<CR>\<C-U>\<Esc>", 'xt')
   call assert_equal('', getline(1))
 
+  call setline(1, 'foobar')
+  setl nomodifiable
+  call assert_fails('startinsert', 'E21:')
+
+  call cursor(1, 1)
+  call assert_fails('startinsert!', 'E21:')
+  call assert_equal(1, col('.'))
+
   set backspace&
   bwipe!
 endfunc
 
+" ":startinsert" is ineffective in a terminal window: it must not give an
+" error and with "!" it must not move the cursor.
+func Test_edit_startinsert_in_terminal()
+  CheckFeature terminal
+
+  let buf = Run_shell_in_terminal({})
+
+  " Fill the terminal with text.
+  if has('win32')
+    call feedkeys("dir\<CR>", 'xt')
+  else
+    call feedkeys("ls\<CR>", 'xt')
+  endif
+  call WaitForAssert({-> assert_notequal('', term_getline(buf, 1))})
+
+  " Go to Terminal-Normal mode and put the cursor on a line with text.
+  call feedkeys("\<C-W>N", 'xt')
+  call assert_notequal(0, search('\S', 'w'))
+  call cursor(line('.'), 1)
+
+  startinsert
+  startinsert!
+  call assert_equal(1, col('.'))
+
+  " Clear "restart_edit" in case the commands were not ignored.
+  stopinsert
+
+  call feedkeys("i", 'xt')
+  call StopShellInTerminal(buf)
+  bwipe!
+endfunc
+
 " Test for :startreplace and :startgreplace
 func Test_edit_startreplace()
   new
@@ -1797,6 +1837,19 @@ func Test_edit_startreplace()
   call assert_equal("axyz\tb", getline(1))
   call feedkeys("0i\<C-R>=execute('startreplace')\<CR>12\e", 'xt')
   call assert_equal("12axyz\tb", getline(1))
+
+  call setline(1, 'abc')
+  setl nomodifiable
+  call assert_fails('startreplace', 'E21:')
+  call assert_fails('startgreplace', 'E21:')
+
+  call cursor(1, 1)
+  call assert_fails('startreplace!', 'E21:')
+  call assert_equal(1, col('.'))
+  call cursor(1, 1)
+  call assert_fails('startgreplace!', 'E21:')
+  call assert_equal(1, col('.'))
+
   bw!
 endfunc