Cursor can land before the prompt after calling beginning-of-line

[Julien Palard]

It can be reproduced on Debian bullseye with bash 5.0.11 on urxvt and xterm and 
by a friend on MacOS with bash 3.2.57.

With this procedure I'm able to reproduce it consistently:

- Move ~/.bashrc elsewhere just to start clean
- Start a new terminal (in my case tput cols tells it's 79 columns, beware, the 
bug varies according to the terminal width).
- run `bash --norc` in it, to start clean
- Prompt in my case is `bash-5.0$ `, beware, the bug varies according to the 
length of the prompt.
- type `printf "Hello World\n                    "` (1)
- hit the `uparrow` of your keyboard to see the printf again (2)
- hit C-a (bash shortcut for beginning-of-line) (3)

After (1) you should see (I'm using ■ to mark the place of the cursor) which is 
OK:

    bash-5.0$ printf "Hello World\n
    Hello World
                        bash-5.0$ ■

After (2) you should see (still OK):

    bash-5.0$ printf "Hello World\n                    "
    Hello World
                        bash-5.0$ printf "Hello World\n                    "■

After (3) you should see (not OK):

    bash-5.0$ printf "Hello World\n                    "
    Hello World
              ■         bash-5.0$ printf "Hello World\n                    "

The cursor should not go that far, it should stop on the `p` of `printf`.

I straced and played a bit with, and noted a few interesting things:

- Bug appear, or not, depending on the length of the prompt (happen with short 
prompts, bug disapear with long prompts)
- Bug appear, or not, depending on the length of the command (happend with long 
commands, bug disapear with short commands)
- Bug appear, or not, depending on the width of the terminal
- When beginning-of-line calls `write(2, "\r\33[C\33[C\33[C\33[C\33[C...` I 
have the bug
- When beginning-of-line calls `write(2, "\10\10\10\10\10\10\10\10\10...` I 
don't have the bug

It can be reproduced without using spaces:

    mdk@seraph:~$ printf "Hello World\n01234567890123456789"
    Hello World
    01234567890123456789mdk@seraph:~$ printf "Hello World\n01234567890123456789"
                  ^ The point jumps here (on the 2nd 4)


Cross-posted here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=948343
-- 
Julien Palard
https://mdk.fr