vim-patch:8.2.3467: CursorHoldI event interferes with "CTRL-G U"

Problem:    CursorHoldI event interferes with "CTRL-G U". (Naohiro Ono)
Solution:   Restore the flag for "CTRL-G U" after triggering CursorHoldI.
            (closes vim/vim#8937)

5a9357d0bf

Co-authored-by: Bram Moolenaar <Bram@vim.org>
(cherry picked from commit a5445f5435)

src/nvim/edit.c

@@ -876,6 +876,10 @@ static int insert_handle_key(InsertState *s)
 
   case K_EVENT:       // some event
     state_handle_k_event();
+    // If CTRL-G U was used apply it to the next typed key.
+    if (dont_sync_undo == kTrue) {
+      dont_sync_undo = kNone;
+    }
     goto check_pum;
 
   case K_COMMAND:       // some command

test/old/testdir/test_autocmd.vim

@@ -103,6 +103,22 @@ if has('timers')
     set updatetime&
   endfunc
 
+  func Test_cursorhold_insert_ctrl_g_U()
+    au CursorHoldI * :
+    set updatetime=20
+    new
+    call timer_start(100, { -> feedkeys("\<Left>foo\<Esc>", 't') })
+    call feedkeys("i()\<C-g>U", 'tx!')
+    sleep 200m
+    call assert_equal('(foo)', getline(1))
+    undo
+    call assert_equal('', getline(1))
+
+    bwipe!
+    au! CursorHoldI
+    set updatetime&
+  endfunc
+
   func Test_OptionSet_modeline()
     CheckFunction test_override
     call test_override('starting', 1)