vim-patch:9.0.1036: undo misbehaves when writing from an insert mode mapping

Problem:    Undo misbehaves when writing from an insert mode mapping.
Solution:   Sync undo when writing. (closes vim/vim#11674)

3f8f827723

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/edit.c

@@ -908,6 +908,12 @@ check_pum:
       }
       pum_want.active = false;
     }
+
+    if (curbuf->b_u_synced) {
+      // The K_EVENT, K_COMMAND, or K_LUA caused undo to be synced.
+      // Need to save the line for undo before inserting the next char.
+      ins_need_undo = true;
+    }
     break;
 
   case K_HOME:        // <Home>