vim-patch:8.2.3671: restarting Insert mode in prompt buffer too often

Problem:    Restarting Insert mode in prompt buffer too often when a callback
            switches windows and comes back. (Sean Dewar)
Solution:   Do not set "restart_edit" when already in Insert mode.
34c20ff85b

As Test_prompt_switch_windows is skipped, implement it in prompt_buffer_spec.

Replace the 50ms term_wait calls with poke_eventloop (test seems to work anyway
without them, so maybe they're not required?)

The new test does include a duplicate screen test that may generate a "screen
test succeeded immediately" warning, but this is done to match the Vim test.

src/nvim/testdir/test_prompt_buffer.vim

@@ -41,6 +41,10 @@ func WriteScript(name)
 	\ '  set nomodified',
 	\ 'endfunc',
 	\ '',
+	\ 'func SwitchWindows()',
+	\ '  call timer_start(0, {-> execute("wincmd p|wincmd p", "")})',
+	\ 'endfunc',
+	\ '',
 	\ 'call setline(1, "other buffer")',
 	\ 'set nomodified',
 	\ 'new',
@@ -103,6 +107,28 @@ func Test_prompt_editing()
   call delete(scriptName)
 endfunc
 
+func Test_prompt_switch_windows()
+  throw 'skipped: TODO'
+  call CanTestPromptBuffer()
+  let scriptName = 'XpromptSwitchWindows'
+  call WriteScript(scriptName)
+
+  let buf = RunVimInTerminal('-S ' . scriptName, {'rows': 12})
+  call WaitForAssert({-> assert_equal('cmd:', term_getline(buf, 1))})
+  call WaitForAssert({-> assert_match('-- INSERT --', term_getline(buf, 12))})
+
+  call term_sendkeys(buf, "\<C-O>:call SwitchWindows()\<CR>")
+  call term_wait(buf, 50)
+  call WaitForAssert({-> assert_match('-- INSERT --', term_getline(buf, 12))})
+
+  call term_sendkeys(buf, "\<Esc>")
+  call term_wait(buf, 50)
+  call WaitForAssert({-> assert_match('^ *$', term_getline(buf, 12))})
+
+  call StopVimInTerminal(buf)
+  call delete(scriptName)
+endfunc
+
 func Test_prompt_garbage_collect()
   func MyPromptCallback(x, text)
     " NOP

src/nvim/window.c

@@ -2271,8 +2271,10 @@ void entering_window(win_T *const win)
   }
 
   // When entering the prompt window restart Insert mode if we were in Insert
-  // mode when we left it.
-  restart_edit = win->w_buffer->b_prompt_insert;
+  // mode when we left it and not already in Insert mode.
+  if ((State & INSERT) == 0) {
+    restart_edit = win->w_buffer->b_prompt_insert;
+  }
 }
 
 /// Closes all windows for buffer `buf`.