patch 9.1.0040: issue with prompt buffer and hidden buffer

Problem:  Modifying a hidden buffer still interferes with prompt buffer
          mode changes.
Solution: Save and restore b_prompt_insert.
          (zeertzjq)

closes: #13875

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
Modifying hidden buffer still interferes with prompt buffer mode changes
diff --git a/src/testdir/test_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index 81a293d..c5ef010 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -297,9 +297,10 @@
   call StopVimInTerminal(buf)
 endfunc
 
-" Modifying a hidden buffer while closing a prompt buffer should not prevent
-" stopping of Insert mode.
-func Test_prompt_close_modify_hidden()
+" Modifying a hidden buffer while leaving a prompt buffer should not prevent
+" stopping of Insert mode, and returning to the prompt buffer later should
+" restore Insert mode.
+func Test_prompt_leave_modify_hidden()
   call CanTestPromptBuffer()
 
   let script =<< trim END
@@ -309,22 +310,34 @@
       new prompt
       set buftype=prompt
 
+      inoremap <buffer> w <Cmd>wincmd w<CR>
       inoremap <buffer> q <Cmd>bwipe!<CR>
-      autocmd BufWinLeave prompt call setbufline('hidden', 1, 'Test')
+      autocmd BufLeave prompt call appendbufline('hidden', '$', 'Leave')
+      autocmd BufEnter prompt call appendbufline('hidden', '$', 'Enter')
+      autocmd BufWinLeave prompt call appendbufline('hidden', '$', 'Close')
   END
-  call writefile(script, 'XpromptCloseModifyHidden', 'D')
+  call writefile(script, 'XpromptLeaveModifyHidden', 'D')
 
-  let buf = RunVimInTerminal('-S XpromptCloseModifyHidden', {'rows': 10})
+  let buf = RunVimInTerminal('-S XpromptLeaveModifyHidden', {'rows': 10})
   call TermWait(buf)
 
   call term_sendkeys(buf, "a")
   call WaitForAssert({-> assert_match('-- INSERT --', term_getline(buf, 10))})
 
+  call term_sendkeys(buf, "w")
+  call WaitForAssert({-> assert_notmatch('-- INSERT --', term_getline(buf, 10))})
+
+  call term_sendkeys(buf, "\<C-W>w")
+  call WaitForAssert({-> assert_match('-- INSERT --', term_getline(buf, 10))})
+
   call term_sendkeys(buf, "q")
   call WaitForAssert({-> assert_notmatch('-- INSERT --', term_getline(buf, 10))})
 
   call term_sendkeys(buf, ":bwipe!\<CR>")
-  call WaitForAssert({-> assert_equal('Test', term_getline(buf, 1))})
+  call WaitForAssert({-> assert_equal('Leave', term_getline(buf, 2))})
+  call WaitForAssert({-> assert_equal('Enter', term_getline(buf, 3))})
+  call WaitForAssert({-> assert_equal('Leave', term_getline(buf, 4))})
+  call WaitForAssert({-> assert_equal('Close', term_getline(buf, 5))})
 
   call StopVimInTerminal(buf)
 endfunc