vim-patch:8.2.1781: writing to prompt buffer interferes with insert mode

Problem:    Writing to prompt buffer interferes with insert mode.
Solution:   Use win_enter() instead of just setting "curwin". (Ben Jackson,
            closes vim/vim#7035)
4537bcc889

Vim test will be skipped, so add a Lua test.
The problem boils down to the use of aucmd_restbuf in a callback, so just test
that (via nvim_buf_set_lines).

src/nvim/autocmd.c

@@ -1208,7 +1208,7 @@ win_found:
 
     win_T *const save_curwin = win_find_by_handle(aco->save_curwin_handle);
     if (save_curwin != NULL) {
-      curwin = save_curwin;
+      win_enter(save_curwin, true);
     } else {
       // Hmm, original window disappeared.  Just use the first one.
       curwin = firstwin;

src/nvim/testdir/test_prompt_buffer.vim

@@ -186,4 +186,38 @@ func Test_prompt_buffer_getbufinfo()
   %bwipe!
 endfunc
 
+function! Test_prompt_while_writing_to_hidden_buffer()
+  throw 'skipped: TODO'
+  call CanTestPromptBuffer()
+  CheckUnix
+
+  " Make a job continuously write to a hidden buffer, check that the prompt
+  " buffer is not affected.
+  let scriptName = 'XpromptscriptHiddenBuf'
+  let script =<< trim END
+    set buftype=prompt
+    call prompt_setprompt( bufnr(), 'cmd:' )
+    let job = job_start(['/bin/sh', '-c',
+        \ 'while true;
+        \   do echo line;
+        \   sleep 0.1;
+        \ done'], #{out_io: 'buffer', out_name: ''})
+    startinsert
+  END
+  eval script->writefile(scriptName)
+
+  let buf = RunVimInTerminal('-S ' .. scriptName, {})
+  call WaitForAssert({-> assert_equal('cmd:', term_getline(buf, 1))})
+
+  call term_sendkeys(buf, 'test')
+  call WaitForAssert({-> assert_equal('cmd:test', term_getline(buf, 1))})
+  call term_sendkeys(buf, 'test')
+  call WaitForAssert({-> assert_equal('cmd:testtest', term_getline(buf, 1))})
+  call term_sendkeys(buf, 'test')
+  call WaitForAssert({-> assert_equal('cmd:testtesttest', term_getline(buf, 1))})
+
+  call StopVimInTerminal(buf)
+  call delete(scriptName)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

test/functional/legacy/prompt_buffer_spec.lua

@@ -1,9 +1,12 @@
 local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
-local feed= helpers.feed
+local feed = helpers.feed
 local source = helpers.source
 local clear = helpers.clear
 local feed_command = helpers.feed_command
+local poke_eventloop = helpers.poke_eventloop
+local meths = helpers.meths
+local eq = helpers.eq
 
 describe('prompt buffer', function()
   local screen
@@ -150,4 +153,13 @@ describe('prompt buffer', function()
     ]])
   end)
 
+  it('keeps insert mode after aucmd_restbuf in callback', function()
+    source [[
+      let s:buf = nvim_create_buf(1, 1)
+      call timer_start(0, {-> nvim_buf_set_lines(s:buf, -1, -1, 0, ['walrus'])})
+      startinsert
+    ]]
+    poke_eventloop()
+    eq({ mode = "i", blocking = false }, meths.get_mode())
+  end)
 end)