Fix 'this-command-keys' wrt prefix argument

* src/keyboard.c (command_loop_1): Restore the feature whereby C-u
was part of this-command-keys, but not of this-single-command-keys.
(Bug#22107)

* lisp/simple.el (internal-echo-keystrokes-prefix): Add
commentary about the function's return value.

2 files changed, 11 insertions, 6 deletions

diff --git a/lisp/simple.el b/lisp/simple.el
index 17b9c918ead..9e0388efe45 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -3818,7 +3818,9 @@ see other processes running on the system, use `list-system-processes'."
 (setq prefix-command--last-echo nil)
 
 (defun internal-echo-keystrokes-prefix ()
-  ;; BEWARE: Called directly from the C code.
+  ;; BEWARE: Called directly from C code.
+  ;; If the return value is non-nil, it means we are in the middle of
+  ;; a command with prefix, such as a command invoked with prefix-arg.
   (if (not prefix-command--needs-update)
       prefix-command--last-echo
     (setq prefix-command--last-echo
diff --git a/src/keyboard.c b/src/keyboard.c
index 02bc7d2a0b7..928d8496cdb 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1486,11 +1486,14 @@ command_loop_1 (void)
       if (!CONSP (last_command_event))
         kset_last_repeatable_command (current_kboard, Vreal_this_command);
 
-      this_command_key_count = 0;
-      this_single_command_key_start = 0;
-
-      if (current_kboard->immediate_echo
-          && !NILP (call0 (Qinternal_echo_keystrokes_prefix)))
+      /* Don't reset this_command_key_count if we've processed
+         prefix-arg.  */
+      if (NILP (call0 (Qinternal_echo_keystrokes_prefix)))
+        {
+          this_command_key_count = 0;
+          this_single_command_key_start = 0;
+        }
+      else if (current_kboard->immediate_echo)
         {
           current_kboard->immediate_echo = false;
           /* Refresh the echo message.  */