vim-patch:8.1.0792: bad display if opening cmdline window from Insert completion

src/nvim/edit.c

@@ -7132,11 +7132,17 @@ static void ins_reg(void)
    * message for it. Only call it explicitly. */
   ++no_u_sync;
   if (regname == '=') {
-    /* Sync undo when evaluating the expression calls setline() or
+    pos_T curpos = curwin->w_cursor;
-     * append(), so that it can be undone separately. */
+
+    // Sync undo when evaluating the expression calls setline() or
+    // append(), so that it can be undone separately.
     u_sync_once = 2;
 
     regname = get_expr_register();
+
+    // Cursor may be moved back a column.
+    curwin->w_cursor = curpos;
+    check_cursor();
   }
   if (regname == NUL || !valid_yank_reg(regname, false)) {
     vim_beep(BO_REG);

src/nvim/ex_getln.c

@@ -51,6 +51,7 @@
 #include "nvim/option.h"
 #include "nvim/os_unix.h"
 #include "nvim/path.h"
+#include "nvim/popupmnu.h"
 #include "nvim/regexp.h"
 #include "nvim/screen.h"
 #include "nvim/search.h"
@@ -6051,7 +6052,12 @@ static int open_cmdwin(void)
 
   /* Don't execute autocommands while creating the window. */
   block_autocmds();
-  /* don't use a new tab page */
+
+  // When using completion in Insert mode with <C-R>=<C-F> one can open the
+  // command line window, but we don't want the popup menu then.
+  pum_undisplay(true);
+
+  // don't use a new tab page
   cmdmod.tab = 0;
   cmdmod.noswapfile = 1;