vim-patch:9.1.1337: Undo corrupted with 'completeopt' "preinsert" when switching buffer (#33600)

Problem:  Undo corrupted with 'completeopt' "preinsert" when switching
          buffer or window.
Solution: Do not delete preinsert text when switching buffer or window.
          (zeertzjq)

related: neovim/neovim#33581
closes: vim/vim#17193

1343681aba

test/old/testdir/test_ins_complete.vim

@@ -3223,7 +3223,7 @@ func Test_complete_info_completed()
   set cot&
 endfunc
 
-function Test_completeopt_preinsert()
+func Test_completeopt_preinsert()
   func Omni_test(findstart, base)
     if a:findstart
       return col(".")
@@ -3363,6 +3363,47 @@ function Test_completeopt_preinsert()
   call assert_equal(4, g:col)
   call assert_equal("wp.", getline('.'))
 
+  %delete _
+  let &l:undolevels = &l:undolevels
+  normal! ifoo
+  let &l:undolevels = &l:undolevels
+  normal! obar
+  let &l:undolevels = &l:undolevels
+  normal! obaz
+  let &l:undolevels = &l:undolevels
+
+  func CheckUndo()
+    let g:errmsg = ''
+    call assert_equal(['foo', 'bar', 'baz'], getline(1, '$'))
+    undo
+    call assert_equal(['foo', 'bar'], getline(1, '$'))
+    undo
+    call assert_equal(['foo'], getline(1, '$'))
+    undo
+    call assert_equal([''], getline(1, '$'))
+    later 3
+    call assert_equal(['foo', 'bar', 'baz'], getline(1, '$'))
+    call assert_equal('', v:errmsg)
+  endfunc
+
+  " Check that switching buffer with "preinsert" doesn't corrupt undo.
+  new
+  setlocal bufhidden=wipe
+  inoremap <buffer> <F2> <Cmd>enew!<CR>
+  call feedkeys("i\<C-X>\<C-O>\<F2>\<Esc>", 'tx')
+  bwipe!
+  call CheckUndo()
+
+  " Check that closing window with "preinsert" doesn't corrupt undo.
+  new
+  setlocal bufhidden=wipe
+  inoremap <buffer> <F2> <Cmd>close!<CR>
+  call feedkeys("i\<C-X>\<C-O>\<F2>\<Esc>", 'tx')
+  call CheckUndo()
+
+  %delete _
+  delfunc CheckUndo
+
   bw!
   set cot&
   set omnifunc&